Rollnir

Weapon (mace), artifact (requires attunement)

When Thor grew tired of excursions into Jotunheim, when Loki was safely bound in the entrails of his children, when Hel was happy in her domain and Ragnarok lay eons into the future, Thor grew bored. And took up TTRPGs.

During centuries of play Mjölnir slowly transformed into a new, even more daunting weapon. Eitri and Brokkr, the svartaelfin brothers who originally fashioned Mjölnir could not explain it, but a spirit of playfulness now infuses the weapon.

Rollnir is a magic, sentient mace that grands a +3 bonus to attack and damage rolls made with it. Its alignment is True Neutral, and it deals an extra 2d6 damage to any non-neutral (LG, LE, CG, CE) target.

Spirit of Rollnir. While in possession of Rollnir you use the "Proficiency Dice" variant presented on DMG p.263.

Sentience. Rollnir is a sentient true neutral weapon with an Intelligence of 11, a Wisdom of 13, and a Charisma of 6. It has hearing and darkvision out to a range of 120 feet.

The weapon can communicate telepathically with its wielder and can speak, read, and understand Common. However, it very rarely chooses to communicate using any of these means. Rather, those wishing to communicate with Rollnir ask it a question and toss it in the air, taking the numbered-face that lands up as an answer, interpreting it as they may. Some claim that the rarity of 8, 16, 3, 17, and 10 "answers" holds great meaning; others decry them as "unbalanced" heretics blaspheming Rollnir's true neutrality. Cries of "ignore priors at your peril" and "past performance is no prediction of future results" are typically flung between the two camps; suggestions of a "saltwater float" have been decried by both branches and "floaters" summarily drowned.

Temperamental. On a turn when you make an attack, first flip a coin. Heads: you have advantage on the first attack you make this turn. Tails: you have disadvantage on the first attack you make this turn.

After this first attack, roll a d20 and apply the indicated "Rollnir Effect."

Rollnir Effects

d20	Effect
1-2	Attacker's face is blackened by a small explosion.
3	Colorful bubbles come out of the attacker's mouth instead of words for one turn. (Individual words are heard as bubbles pop, in random order.) Attacker cannot speak effectively during this time.
4	Target's feet enlarge, reducing their movement to half for 1d3 turns.
5	Target ages 10 years.
6-20	Rollnir is Temperamental for the next attack it makes.

Credit: @Ben (Stack Exchange chatizen), possibly