Spellthief (class)

Spellthieves are meant to combine - or at least, make a compromise between - the magic of and the stealthiness and speed of, which may come off as either successful or not. After all, magic has powers that may grant people invisibility and the ability pull and push objects from afar... But magic also has a nasty tendency to be flashy and noisy, which is rather counterproductive to those who really need to be stealthy.

This is why Spellthieves rely on mostly Utility Magic while not in combat, and restrict non-utility spells to combat use only, mostly as a backup once silent takedowns cease being an option.

They use Arcane Magic, just like.

The Original FRPG
Spellthieves make no overt appearance in the original FRPG, as they are an innovation introduced in the game adaptations - however, dual-classed Thief-Magician hybrids were basically Spellthieves in all but name.

RPG Maker Adaptations: 2012 and 2013
In the RPG Maker-based video game adaptations of the FRPG, Spellthieves - just like their names suggest - are a compromise between and. They are fast, have a somewhat wider selection of weapons than Magicians, can cast spells, but are not as good at Magic as Magicians, and are slower and weaker than regular Thieves.

World of Artograch
The tabletop roleplaying system does away with classes altogether, deprecating them in favour of backgrounds, which allow players more flexibility during character creations. Nevertheless, it's still very much possible to play as a Spellthief - one simply has to compromise Magic with Stealth feats, which is very much possible with any of the Magic-oriented and Stealth-oriented backgrounds, or with no background at all.