It is impossible to reach the pinnacle of skill without a little magic. You don't have time to study, and you know better than to make a deal with the devil, so instead you have learned to steal the magic you need to better yourself. As a spellthief, you are versatile and innovative, never casting two spells when one will do. Your desire to prove yourself underlines most of what you do; it is this desire which led you to seek magic in the first place, and you view each challenge as a new chance to prove your mastery.
When you reach 3rd level, you gain the ability to cast spells.
Cantrips. You learn two cantrips of your choice from the sorcerer spell list. You learn another sorcerer cantrip of your choice at 11th level.
Spell Points. You do not gain any spell slots. When you use a melee attack to deliver your sneak attack, you may instead forego a number of dice of sneak attack damage. For each die you forego, you gain a spell point.
You may then expend a number of spell points to create a spell slot of a given level, and use that slot to cast a spell. You can't reduce your spell point total to less than 0. You cannot create a spell slot of a level higher than your Maximum Spell level.
You may gain a number of spell points equal to the number shown under the Stolen Spell Points column of the Spellthief Spellcasting table. Casting a spell with stolen spell points does not allow you to steal additional points. You regain the ability to steal spell points when you complete a short or long rest, and any unused stolen spell points are lost at that time.
Spells Known of 1st-level And Higher. You know one 1st-level sorcerer spell of your choice, which must be selected from the necromancy, or conjuration spells on the sorcerer spell list.
The Spells Known column of the Spellthief Spellcasting table shows when you learn more sorcerer spells of 1st level or higher. Each of these spells must be a necromancy or conjuration spell of your choice, and must be a level no higher than what's shown in the table's Maximum Spell Level column for your level. For instance, when you reach 7th level in this class, you can learn one new spell of 1st or 2nd level.
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 no higher than your maximum Spell Level, and it must be a necromancy or conjuration spell.
Spellcasting Ability. Charisma is your spellcasting ability for your sorcerer spells, since you learn your spells through intuition and theft. You use your Charisma 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.
Spell save DC = 8 + your proficiency bonus +
your Charisma modifier
Spell attack modifier = your proficiency bonus +
your Charisma modifier
Spellcasting Focus. You can use an arcane focus as a spellcasting focus for your sorcerer spells.
Starting at 3rd level, as an action, you can attempt to borrow knowledge of a spell from another creature. You must physically touch the creature, and they must be non-hostile towards you or incapacitated.
The target must make a Charisma save against your Spellthief saving throw DC. On a failed save, you may select one spell the creature knows (or has prepared, if they prepare spells) of a level no higher than your maximum spell level, provided the spell is at least 1st level. The spell does not have to be a Sorcerer spell. If the target does not know the spell you are trying to borrow, this ability fails and the action is wasted.
As long as the spell is borrowed, you can cast it using your stolen spell slots as if it were a sorcerer spell, but you do not know the spell. You retain a borrowed spell until you choose to forget it, at which point you may attempt to borrow a new spell.
The Borrowed Knowledge column on the Spellthief Spellcasting table shows how many spells you may have borrowed at your level.
When you reach 9th level, you are able to discern the spell knowledge of other creatures. As an action, make a melee spell attack against a creature you can touch. If you succeed, you learn the name of every spell your victim has prepared (if they prepare spells) or knows (if they do not) up to your slot level.
When you reach 13th level, you may choose to borrow skill or tool proficiencies, in addition to spells. The target of your borrowed knowledge ability must be proficient in the skill or tool you wish to borrow, and you must have a borrowed knowledge slot available. Instead of borrowing a new spell, you instead gain proficiency in the skill or tool chosen. This skill or tool uses one of your borrowed knowledge slots as if it were a spell.
At 17th level, you are able to take control of another caster's spell at the moment of its inception. As a reaction, which you take when you see a creature within 60 feet of you casting a spell, you may attempt to wrest control of the spell being cast. Make an ability check using your spellcasting ability. The DC equals 10 + the level of the spell being cast. On a success, the caster fails to cast the spell, and you immediately cast the spell as if you were the original caster. The new spell uses your spellthief spell save DC or spell attack modifier, and you are the origin of the spell for the purpose of range and line of sight.
Once you have used this ability, you can't use it again until you finish a long rest.