Spellthief

The Spellthief is a character class from the 3.5 edition of D&D, printed in the Complete Adventurer.

It is a hybrid class, combining many class features from the Rogue such as BAB, Sneak Attack and Trapfinding with some CHA-based spontaneous spellcasting from the Wizard/Sorcerer spell lists.

While this sounds a bit like a Bard variant, it is far from it. As mentioned, the ability to choose and cast spells from the Wizard/Sorcerer list is practically golden, even though you only achieve this up to 4th level spells.

Class features unique to the Spellthief include the ability to "Steal Spells" which allows the PC to forego some sneak attack damage to remove a prepared spell from a targets memory (spontaneous casters lost the ability to cast it for a minute) and allow them to store it themselves and cast it later on. Either that or they can mooch spells off of willing party members with a touch.

This gets especially handy as the class feature scales with the class level, eventually up to 9th level casting ability much later in their progression. There is no limit to the number of times this can be performed each day, only the number of spell levels being stored, therefore the Spellthief can theoretically match high-level dedicated casters and continue casting for longer each day so long as they can keep stealing spells.

Of course there are caveats to this: it means finding both an appropriate target and getting into a position where they can steal spells, meaning a lot of focus on sneak attacking. Also the stored energy must be used within a certain time period, which is 1-hour for stolen spells. Later they can also steal active spell effects and spell-like-abilities from targets, which last for an even smaller amount of time before they disperse. Finally, the Spellthief must either know which spell he wants to attempt to steal, or the DM allocates it randomly, but that gets mitigated later on when the class learns how to "discover spells" when they use the ability on a target.

Cool features later though are the ability to absorb incoming spells, which is a simple d20 + level check vs caster level + 10, meaning that there is about a 50% or so chance of simply negating incoming spells that specifically target a dedicated spellthief against opponents of equivalent level.

Making Spellthieves Work

The Spellthief class is one that works well right up to 20th level, with some of the best abilities coming later in its career.

Multi-classing out of the class means that you'll likely lose the ability to absorb incoming spells, and will heavily reduce your ability to steal high level spells from willing/unwilling targets.