Sorcerous Origin: Spell Thief

Not all sorcerers were born with magic, some recived it thought experiments or other man-made methods. These sorcerers did not gain the same capability to shape the weave as a wizard or cleric. Instead, these sorcerers gained the ability to disrupt and turn other mage's magic against them. Known to the few unlucky enough to encounter them as Mage Banes or Spell Thieves, they seem to have an uncanny eye and understanding of how to rip other mage's weave to shreds and use it against them.

Eye on the Prize

At 1st level you have an innate keen understanding of the magic around you, and are able to understand a mage's intent as they cast. You can attempt to identify a spell without using your reaction. When you make an ability check to identify a spell as it's being cast, you gain the following benefits:

• You are considered proficient in the Arcana skill for the roll, and add your proficiency bonus twice instead of once.
• You can use Charisma instead of Intelegen.

Weave Siphon

Additonally at 1st level, you can siphon some of the excess magic from a spell to fuel your own endevours. When another creature casts a spell of 1st level or higher within 60 feet, you can use your reaction to regain a number of Sorcery Points equal to half the spell level rounded up, with a minimum of one. You can use this feature a number of times equal to your Proficiency Bonus, recharging a long rest.

Arcane Vandal

At 6th level you learn Counterspell and Dispel Magic, and they don't count towards the max number of sorcerer spells known. You cast either of these spells by expending 2 Sorcery Points, you can upcast the spells, costing 1 additional sorcery point per level above 3rd. If you successfully counter or dispel a spell, you can steal it.

When you steal a spell, you learn the spell, if it is not a part of the sorcerer spell list, it counts as a sorcerer spell for you. You can know a number of stolen spells equal to your Proficiency Bonus, if you want to use this feature on a spell while already having the max number of stolen spells you can replace one of your stolen spells with the new one.

Weave Gazer

additionally at 6th level, your understanding of the weave around you grows ever sharper. When you use Eye on the Prize, You also learn the level the spell is being cast at. Additionally when you make an ability check using your spellcasting ability, you can add your Proficiency Bonus

Sorcerous Reversal

At 14th level, you can further manipulate others magic, bending it to your whim instead of just countering it. If you sucessfully counter or dispell a spell you can as part of the same reaction, choose a new target for the spell, instead of countering it, including the caster. when you do this you can not steal the spell.

Arcane Sabotage

At 18th level, you learn how to truely remove magic from others. If you sucessfully counter or dispell a spell and steal it, you can lock it from the caster, making them unable to cast the spell until you replace it from your stolen spells. Once you use this feature, you can't use it again until you finish a short or long rest.

Credit: Thief (2014)

Identifying spells (XGtE p.82)

Sometimes a character wants to identify a spell that someone else is casting or that was already cast. To do so, a character can use their reaction to identify a spell as it's being cast, or they can use an action on their turn to identify a spell by its effect after it is cast. If the character perceived the casting, the spell's effect, or both, the character can make an Intelligence (Arcana) check with the reaction or action. The DC equals 15 + the spell's level. If the spell is cast as a class spell and the character is a member of that class, the check is made with advantage. For example, if the spellcaster casts a spell as a cleric, another cleric has advantage on the check to identify the spell. Some spells aren't associated with any class when they're cast, such as when a monster uses its Innate Spellcasting trait.