Or a circumstantial "Summon" spell, that would summon a different creature based on what it is cast upon.
Imp Dust -> Imp
Wisp Dust -> Wisp
Unicorn Horn -> Nightmare
Tree -> Ent
Stone -> Golem
The "Reanimate" spell would also be much more interesting if it were circumstantial, generating a different type of undead based on the corpse used.