Spellthief

Roguish Archetype
Comments from the Finger: Literally the most requested subclass ever, right here. 

**I completely forgot to credit Casey Williams, who collaborated with me on this one! I’m sorry Casey!**

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.

Spellthief Spellcasting Table

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.

Spell save DC = 8 + your proficiency bonus + your Charisma modifier

Spell attack modifier = your proficiency bonus + your Charisma modifier

Spell Stealing
Starting at 3rd level, when you Sneak Attack a creature that has the Spellcasting or Innate Spellcasting features, you can forgo half Sneak Attack damage dice (rounded down) to siphon away their arcane energy. If you do so, you gain a single filled spell slot of the level you are capable of casting and you temporarily learn one spell of your choice that the creature was capable of casting. This spell must be a level for which you have spell slots. The spell slot you gain and the knowledge of this spell last until you complete long rest. The creature also loses its spell slot, or one use of a spell it is capable of casting, as appropriate.
     The spell slot you gain with this feature increases in level as you gain levels, as shown in the Spell Level column of the Spellthief Spellcasting Table.
     As a bonus action, you can also take magic from a willing creature within your reach that has the Spellcasting or Innate Spellcasting features. When you do so, that creature expends spell slots whose combined level is greater than or equal to your spell slot level, and you regain an expended spell slot. You can also learn a spell that the creature was capable of casting until you complete a long rest.

Arcane Sight
By 9th level, you can innately detect magical auras. You can cast the spell detect magic as a bonus action without expending a spell slot.

Spellgrace
At 13th level, you can add your proficiency bonus to all saving throws you make against spells that don’t already add your proficiency bonus.

Total Conversion
By 17th level, when you steal a spell, you not only gain a new spell slot, but you also can regain an expended spell slot. Moreover, your target can’t cast the spell that you learn from them for 1 minute.

Changelog: 9/22/16: Spellgrace: Clarified that you don’t add your proficiency bonus twice