The Spellthief is a character class from the 3.5 edition of D&D, printed in the Complete Adventurer.
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.
- 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.
However, the downside to all of this is obvious. It's hinging its usefulness on what amounts to a single very situational trick, despite all of the variations it gets as the class progresses. Its reliance on other spellcasters makes the class virtually useless in any other situation, since the class features are so heavily focusses on one thing.
While they have pretty decent skill points and a good range of skills, they don't really do well as replacements for a party's Rogue, nor should they ever be considered replacements for the party spellcaster. And if they have to lower themselves to mooching spells off of the party wizard/cleric then the party hasn't actually gained anything useful and the player playing the wizard/cleric will wonder what was the point.
Making Spellthieves Work
As mentioned Spellthief is a bit of a one-trick pony, but it isn't an absolutely terrible class. If the campaign is very magic focused (emphasis on very), then the Spellthief can be a decent and somewhat satisfying choice for a PC. The Spellthief class is one that works well right up to 20th level, with some of the best abilities coming later in its career.
In other situations where magical opponents are not so prevalent then it might have been better to play a Rogue, or if the player is dead set on playing an Arcane Rogue, then be a Beguiler instead.
No other class, prestige or otherwise has the Steal Spell ability that synergises with the Spellthief, (yeah Ur-Priests can steal spell-like abilities, but whatever) therefore 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.
Therefore someone insistent on playing a Spellthief should focus on feats that improve their sneak attack, stealth ability or overall mobility, making them more likely to use their class feature. You can also select spells that work well with sneak attack, for example Golem Strike is a spell that lets you sneak attack constructs (which sadly on its own only lasts for one turn).
That said... it makes a nice filler level for players wanting to introduce a magical element to their own abilities. The low level class features work no matter what the power level of your group is, as level-0 or level-1 spells will remain fairly common while sneak attack and trapfinding stack with class abilities elsewhere so is good for Rogues waiting to qualify for their own Prestige Classes or basically just anyone who wants to introduce a bit of Rogue-like skill and magical mischief into their own character without having to hinder their progression too much.