The Spellthief

Spellthieves weave skill with arcana to drain the magic of their foes and turn their powers against them. Your well of magical energy is shallow compared to that of a wizard or sorcerer, and you lack the brute strength of a fighter, but you have the skills to adapt to any challenge, and the ability to steal the tools necessary to adapt when you are without them.

Spellcasting

When you reach 3rd level, you gain the ability to cast spells. See chapter 10 for the general rules of spellcasting and chapter 11 for the sorcerer spell list.
     Cantrips. You learn two cantrips from the sorcerer spell list. You learn another sorcerer cantrip of your choice at 10th level.
     Spell Slots. You gain two spells slots, and an additional one at 13th level. Your spell slots are all if the same level, and always of the highest level that you can cast, as shown on the Spellthief Spellcasting Table. You regain any expended spell slots when you finish a short or long rest.
     Spells Known. You know three 1st level spells from the sorcerer spells list of your choice. The Spellthief Spellcasting Table shows when you learn more spells of 1st level or higher. Each of these spells must be at a level for which you have spell slot.
     Whenever you gain a level in this class, you can replace one of the sorcerer spells you know with another spell of your choice from the sorcerer spell list. The new spell must be of a level for which you have spell slots.
     Spellcasting Ability. Charisma is your spellcasting ability for your sorcerer spells, since you command the power of magic though your force of personality. Use your Charisma score whenever a spell refers to your spellcasting ability. In addition, you use your Charisma modifier when setting the saving throw DC for a sorcerer spell you cast and when making an attack roll with one.

Spellstealing

Beginning when you choose this archetype at 3rd level, you can siphon magical ability away from your target and use it for yourself. When you hit an opponent with a successful sneak attack you can choose to forgo dealing sneak attack damage and instead attempt to steal a spell from the target. If the target is willing, you can steal a spell with a touch as an action. You can choose which spell to steal if it is known; if the attempted stolen spell is not prepared or on their casting list, the spell siphon fails.

You can steal a spell of a level you have slots for automatically upon hit. You can attempt to steal a spell of a higher level with a successful ability check using your spellcasting ability. The DC equals 10 + the spell level.

The target of a steal spell attack loses the ability to cast the stolen spell. For up to 1 hour, on a subsequent turn, you can cast the spell once as an action without expending a spell slot. Once the spell has been cast or the time expired, the target regains the ability to cast the spell.

For example, at 7th level against a 5th level Sorcerer, you could attempt to steal Fireball, a 3rd level spell. Assuming the Sorcerer knows that spell and has 3rd level slots remaining, you make an Intelligence Check with DC 15. If successful, on a subsequent turn or at any time within 1 hour, you can cast Fireball without expending a spell slot. The sorcerer loses the ability to cast Fireball during that time.

At any one time, you can possess a maximum number of stolen spell levels equal to your class level. For instance, at 4th level you can have two stolen 2nd-level spells, or one 2nd-level spell and two 1st-level spells, or any other combination of spells totaling four levels. If you steal a spell that would exceed this limit, you must choose to lose stolen spells sufficient to reduce the total number of stolen spell levels to no more than your maximum.

Steal Damage Resistance

Beginning at 9th level, you can siphon off a target’s resistance to a damage type. When you hit an opponent with a successful sneak attack you can choose to forgo dealing sneak attack damage and instead temporarily gain resistance to a damage type to which your target is resistant or immune. If the target is willing, you can steal damage resistance with a touch as an action.

Simultaneously, the target creature’s resistance to that damage type is temporarily suppressed. A creature with immunity to a damage type retains that immunity. If your target has more than one type of resistance to damage, you can choose which to steal if it is known; otherwise, the DM determines the stolen resistance randomly from among those possessed by the target. If you choose to steal a type of resistance that the target doesn’t possess, the stolen type of resistance is determined randomly from those possessed by the target.

The resistance you gain from using this ability lasts for 1 minute. If the resistance is derived from a temporary effect (such as a spell), the stolen resistance disappears when the effect expires.

You can use this ability multiple times, but its effects do not stack unless they apply to different types of damage.

For example, throughout a long combat, you might use this ability to gain resistance to fire and cold, but you can not use it twice on a creature that is resistant to fire to gain twice as much resistance to fire (nor to reduce the creature’s resistance to fire by twice as much).

Steal Spell Effect

Starting at 13th level, you can siphon an active spell effect from another creature. When you hit an opponent with a successful sneak attack you can choose to forgo dealing sneak attack damage and instead gain the effect of a single spell affecting the target. If the target is willing, you can steal a spell effect with a touch as an action.

You can choose which spell effect to steal; otherwise, the DM determines the stolen spell effect randomly. If you try to steal a spell effect that isn’t present, the stolen spell effect is determined randomly from among those currently in effect on the target. You can’t steal a spell effect if the spellcaster’s level exceeds your class level + their Charisma modifier.

Upon stealing a spell effect, you gain the stolen effect (and the original creature loses that effect) for 1 minute (or until the spell’s duration expires, whichever comes first). If the spell effect’s duration hasn’t expired by this time, the spell effect returns to the creature that originally benefited from it.

This ability does not work on spell effects that are immune to dispel magic (such as bestow curse).

Absorb Spell

Starting at 17th level, when you are targeted by a spell or in the area of effect of a spell, you can choose to absorb the spell. Doing so negates the effects from the spell and the spell is added to your stolen spell list if possible. The spell is subjected to the same rules as a spell stolen by the spellsteal ability.

This ability can be used once per long rest.