The Corruption of Pelursk

GM's Reference & Directory by Mike Harvey

In the campaign primer, the adventure is referred to as The Curious Case of Black Lake, in an effort to somewhat obscure the module in case of player pre-reading. Ideally players will understand not to look up the adventure ahead of time (and realistically, if they wish to do so, the mention of Pelurskite is another lead to go on), but it is a precautionary step.

This material assumes a party of 4-5 adventurers of 5th level, utilizing character options only from official D&D 5e published material. It is not intended to be a particularly difficult nor deadly adventure, although engaging the Pelurskite King in combat should lead to a tense and complex encounter for the PCs. See below for Running the Pelurskite King Encounter..

Why 5th level?
It gives players a toolbox of options they can use to solve the problems within the adventure, without delving into magic powerful enough to trivialize much of it (eg spells that bestow flight, conjure particularly potent creatures, teleport great distances, etc). 5th level characters are hardier and have substantial enough hit points to face the many hazards of the island. They also have fairly capable ability checks if prepared well.
When the party attempts to cross to the Island, what if the PCs have access to Carpenter's Tools?
A character proficient in Carpenter's Tools could fashion a reasonable, rudimentary raft within a day's time, provided they acquire the wood (not difficult, given the nearby forests). Beginning from scratch, they would need at least a tenday (and proper environment) to construct a proper, sealed watercraft, and in doing so would alert all the villagers to their intents to breach the sacred ground of the Island.
A rudimentary raft constructed by a PC would face similar issues to the vessel the party can acquire - it is unable to seal the water beneath it from splashing the PCs, nor from transferring the intense heat of the lake, without adequate insulation (ie wolf pelts, etc).
What if the PCs want to rest on the island?
The party is free to attempt to take a rest on the island, be it long or short. However, without adequate measures taken, their rest is highly likely to be interrupted by hostile entities spurred on by the will of the Pelurskite King.
When to roll for a Roving Monster Encounter
If the PCs set down to take a short rest anywhere that doesn't offer reasonable shelter/cover/fortifications (which they may be able to construct themselves), designate one character to take watch. That character must make a DC 15 Wisdom (Perception) check. On a failure, roll on the Roving Monster Encounters table as normal, and the PCs encounter the result. On a success, roll twice on the Roving Monsters table, and take the lower result.
If the PCs wish to take a long rest on the island, they will have to put in work to erect a defensible position to ward off attack. Under normal circumstances, only Ruins Encounters hexes are eligible for long rests due to their enduring structures from which a shelter can be fashioned. When setting a camp, one character is designated to take point on the design of the camp; they must make either a Wisdom (Survival) check or an Intelligence (Stealth) check to direct others in constructing a concealable and defensible shelter for the rest. Another character proficient in the skill not being used may assist the designated character to give them advantage on the check. Camp efforts take a minimum of 1 hour to complete.
On a result of 20 or higher, do not roll on the Roving Monsters table; the rest passes uneventfully. On a result of 15-19, roll twice on the Roving Monsters table and take the lower result. On a result of 11-14, roll once on the Roving Monsters table. On a result of 10 or lower, roll twice on the Roving Monsters table and take the higher result.
Generally speaking, if the party is in a hex where they're dealing with an Island Attacks encounter, there is no peace to be had there and resting will fail.
If the party faces a Ghost encounter and overcomes any threat there, they can then safely take a short rest in that hex.
What if the PCs cast Leomund's Tiny Hut?
Characters may have access to the 3rd-level, ritual-tagged spell Leomund's Tiny Hut, allowing them to spend very little time (and possibly no resources) to conjure a 10' dome of otherwise impenetrable force for shelter that lasts a full 8 hours. In theory, the PCs could cast this spell pretty much anywhere on the island and claim a long rest uninterrupted. This is a viable strategy to surviving this adventure and should not be discouraged. However, if the party does not take the same precautions to secure their shelter before casting the spell (as above), their position is forfeit throughout the 8 hour period; when the spell ends, roll on the Island Attacks table as the island takes it's first opportunity to strike at the once more vulnerable PCs. Entry 6, the pelurskite well encounter, may be particularly appropriate as the ground surrounding the 10ft dome falls away to reveal a 15ft radius, 30ft deep ring around them, its walls lined with stinking, steaming hot pelurskite and a well of boiling water at it's base.

If the PCs take long rests on the island and spend more than a day there, what happens?
Due to the nature of the island, the PCs are at all times unable to see the sky above them to denote time of day (save for one or two particular hexes). If they spend more than the one night there, they aren't pursued by the villagers, but must still fend for themselves. By the time the party arrives at Black Lake, their supplies from the trip there should be all but depleted; food and water can be acquired in town for a price. Characters may be able to cast spells such as Create Food and Water, Create or Destroy Water, Goodberry, Purify Food and Drink, etc to nourish themselves in an environment that is almost barren of potable water and edible flora/fauna.
Food and Water Needs
See PHB page 185 for rules on Food and Water. Ideally a character needs 1 pound of food and 1 gallon of water per day. A waterskin holds approximately half a gallon of water, so a single waterskin filled is insufficient for a full day's water needs (make sure players understand this ahead of time). Characters who cannot access potable water on the island will begin accumulating levels of exhaustion quickly if they spend more than a day there.
A character with the Outlander background (or simply the Wanderer feature) will find that the island does not provide the necessary amenities (berries, small game, fresh water, etc). Likewise, because it is constantly shifting to confound and entrap the PCs, the Wanderer is not able to reliably look upon the landmarks and pin down their exact location.
Ranger's Natural Explorer feature: The Island is considered Forest in regards to the Ranger's favoured terrain features. However, two considerations apply: 1) Because the island is constantly shifting to draw the PCs towards the centre, this would be magical means by which the PCs were becoming lost; and 2) foraging by the ranger yielding twice as much food as normal will not help as there is not sufficient plant life and wild life to successfully forage in the first place.
The water of Black Lake is dangerous to consume not only because it is boiling, but also because it is contaminated with unstable essence of corrupted pelurskite. A creature drinking water from Black Lake, which has not been first purified, must succeed on a DC 15 Constitution saving throw or be poisoned for 1 hour. You can refer to Xanathar's Guide to Everything page 79 regarding the use of Brewer's Supplies, as they may be used to purify water (6 gallons as part of a long rest, or 1 gallon as part of a short rest). Identifying the impurities in the water, without imbibing any of it, would require a DC 10 Intelligence (Brewer's Supplies) check.

Using Session Zero

It may be helpful to hold a Session Zero before beginning the game with your group. In fact, holding S0 on a separate date before the adventure may be necessary depending on your time considerations - as formatted and presented here, this adventure could easily take a group between three and five hours, and more than that could be too much to ask of the group - including yourself!

Setting Expectations. The campaign primer is meant to provide adequate information to players to create characters who will be well-suited for the adventure. However, going over the themes, goals, and focus of the adventure actively with your players will create a stronger, mutual understanding and reference point. It also gives your players an opportunity to express the elements they're excited to see in the game, any topics or elements they aren't comfortable with, and how to address those. Discuss how long you foresee the main game session taking. Try to communicate the tone of the adventure - overall dark with horror elements - and consider dissuading players from making characters who won't fit with that sort of game. One-shots can be a great place to try out odd and joke-y character concepts, but you may decide that this is not the right game for that type of character.
Pitching Characters. Your players will have an opportunity to bounces ideas off of one another when it comes to creating characters for this adventure. It also creates an open forum for them to pitch you a concept for their characters, and you to work with them to fine-tune their ideas into a satisfying and intriguing character - you should be a fan of your player's characters, and collaborating to realize those characters will give everybody a more coordinated dynamic. This is also an excellent opportunity to determine if your any of your characters have a relationship prior to this mission and develop that - perhaps two or more characters have worked together in the past; some may be related, or old friends; there may be rivals; perhaps some come from the same faction. The player characters can absolutely have unique links to Mathilda Vodren and come together as a group of strangers, while it can also be rewarding to have some interpersonal links between characters to play out.
Building Characters. Depending upon the game experience of your players, character creation could be quick, or it could stretch out a fair bit (remember that the PCs receive extra starting gold, with which they can shop for additional gear - this can take time!). It could also be longer and more in-depth if your group is excited to collaborate and weave their characters' histories together. Plan for up to an hour of character creation time in S0.
Flexing Mechanics and Roleplay. The party begins the main adventure after twenty-three days travel on horseback, during which time they have had ample opportunity to get to know each other. When they do set out into the adventure proper, it would help if the players had an inkling of what those relationships look like, and what their fellow party members are like! Once characters have been created, set up a fairly simple roadside encounter, where one evening during their trip they are waylaid by a bandit blockade. This would be an excellent opportunity to let the players test out their characters' abilities in combat, but it could also be a chance for them to find another solution to the conflict and roleplay their way out of things - or try to! This will give them a chance to make any minor mechanical tweaks to their characters before the full adventure begins.
See the Roadside Bandit Encounter page to run this encounter.

The Last Night on the Road. Open on a cold fireside camp night, on the last eve of their journey before they should reach Black Lake. Highlight the harsh cold climate here (thick, oppressive cloud cover, drowning the moon in a sea of grey; bitter winds that they’ve learned to dampen by making camp within the tree cover; the dry air this far, as in the last days before and the first days after Citadel Felbar the snow fell freely, but now it covers the ground with nothing falling to replenish it). Remind them their supplies are all but spent, but should get them to the town where they should be able to restock. Go around and ask each player what two things the other PCs would have learned/noticed about their character’s personality over the course of this journey. Maybe what they’re like when things are good, and what sticks out when they’re in a jam. Let them roleplay a bit of campfire carousing and get into character, play off of each other. Maybe suggest somebody have their character tell a story of a quest that went well, and somebody else tell of a quest that went poorly.

Mac Asgrel's Mansion

The party may come to Mac Asgrel's mansion before venturing into the village of Black Lake, as it is closer to them from the road. It is entirely possible that the PCs will get aggressive with Mac Asgrel, particularly if they discover that he keeps a piece of perfect pelurskite around his neck - just the sort of sample they're looking for.

Should rolls become necessary, use this stat block for Mac Asgrel. When he answers the door for the PCs, have one PC make a Charisma check (with relevant skill proficiency depending upon their tact) and roll Mac Asgrel's Wisdom (Insight) against it. If Mac Asgrel's result is higher, his mistrust of the PCs gets the better of him and he closes the door (making a show of doing so to remove the door chain) before casting Mislead, carefully moving away whilst invisible to a safe place so that he may deal with the strangers through his illusory double. If so he will try to quietly lock himself in his bedroom for safety.
If during his dealings with the PCs, Mac Asgrel senses aggression from them, he will cast Mislead if available, and if not, he will cast either Blink or Dimension Door to extricate himself from their clutches and hide away in the woods somewhere until they have left his property. Mac Asgrel will desparately cling onto his pelurskite pendant, but will also not wish to engage the PCs as he can easily tell they are capable combatants and easily outnumber him.
See here for a floor plan of Mac Asgrel's mansion, in the event that your players are interested in exploring it.

Running the Pelurskite King Encounter

The pelurskite dais is comprised of 27 hexagonal white platforms that form steps, leading up to a plateau where the King sits fused to the yellow-and-red throne. Each 5ft. section leading up to the platform contains five 1ft. steps, and these are not difficult terrain. The ground around it is firm earth and is also not difficult terrain.
Eight pelurskite outcroppings jut from the earth around the dais - they are all roughly 5ft. tall and count as blocking terrain. The sections of dense tree cover at the edges/corners of the clearing are blocking terrain, and only the spaces between these sections are traversible if the PCs wish to enter/exit this clearing.
There is a thick, humid, greasy, pale-yellow smog emiting from the throne. The smog covers the area 10ft. above the throne, down to the top of the 15th step up, rendering that area heavily obscured. Creatures and objects on the plateau itself are only lightly obscured from each other, as the smog is billowing down and thinnest here.
On the first round of initiative, roll for the King and have the PCs roll as well. The King will use his action to Multiattack if there is a creature within his reach (any space 5ft. away from the front of the throne, that is the two squares containing arm rests; the back of the throne blocks his reach and is blocking terrain); otherwise he will use Piercing Resonance. If he hits with Multiattack, on the first successful Claws attack he will always forgo the slashing damage to instead grapple the target, allowing him to Bite the target as a bonus action if both hit - also allowing him to Bite a grappled target on a subsequent turn as a Legendary Action.
The King will spend a Legendary Action to make a Wisdom (Perception) check against a hidden PC. A PC is able to hide from the King if they are outside of his blindsight, even if they are within his tremorsense, which will render them untargetable by some of the King's Lair Actions. The King can use his tremorsense in the check to attempt to locate the hidden PC. The lightly obscuring smog around the throne does not impose disadvantage on the King's Wisdom (Perception) check.
On round 2, and on every subsequent even-numbered round, 1d4 Pelurskite Sprites each appear within 5ft. of a different pelurskite outcropping. Roll initiative for the Sprites at the start of round 2, and Sprites both spawn and act on their collective initiative count. They move to engage PCs who are keeping distance from the King and the smog, and favour targets knocked prone by the King's Lair Actions. Sprites stop spawning when the King is defeated, or when there are no remaining outcroppings.
The pelurskite outcroppings can be attacked and destroyed; each has AC 15, 10 HP (resistance to piercing and slashing damage from weapons that aren't adamantine, and immunity to fire and poison damage). A destroyed outcropping cannot spawn Sprites. On the Sprites' initiative, they cannot spawn more Sprites than the number of remaining outcroppings.
Lair Actions. The King uses his Lair Action options as follows:
He uses the column option to push PCs closer to himself, ideally up the dais and within his melee reach if possible. Columns can extend from any space that is pelurskite dais or outcropping, or earth. They cannot extend from the dense tree cover.
He uses the quake option ideally when he has a PC grappled, as it makes it harder for them to escape him; it is also useful to slow PCs trying to retreat from the clearing, and to grant Sprites advantage on their melee attacks.

He uses the pit option against a PC dealing in ranged attacks/magic to render them less effective; he likewise uses it to entrap a PC trying to retreat from the clearing. The King cannot open a pit inside a space that the dais or an outcropping occupy.

Changelog

November 21 2019, first draft complete with reference questions & answers, as well as guidelines for running the Pelurskite King combat encounter. Included links for the adventure shop page, the campaign primer, the Isle of Pelursk rolling tables, the Pelurskite King's stat block, and an image of the battlemap design for the Pelurskite King Encounter.
December 11 2019, added section on running a Session Zero for the campaign, as well as a section for running an encounter with Mac Asgrel. Included links for the Roadside Bandit Encounter and for Mac Asgrel's stat block. Missing a floor plan map of Mac Asgrel's mansion; will add at a later date.
December 15 2019, added a floor plan for Mac Asgrel Manor with a rundown of its layout and features.
December 16 2019, added a note regarding potable water on the Island and the use of Brewer's Supplies (as detailed in XGtE) to purify water from Black Lake.

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