Protos Magos is a game in the FPS genre that I played where the weapons are spells as the game's setting was of a gothic fantasy kind. These spells could be slot into other spells which would make that spell act differently like you could make the regular orb spell spawn a toxic pool by combining it with a toxic spell. 

It was a fun game and I found some really great bugs like I managed to get out of bounds in two spots!