Slayer and Super Slayer Deeds come from completing Monster Kill Contracts. Upon Completion of these contracts there is a small chance you get a Deed as an extra reward. You can add these Slayers to any weapons or spellbook of your own choosing even if they are already magical.
Note that 2 Slayers slots are available meaning you could in example make a Indestructible Vanquishing Exceedingly Accurate Katana of Reptile and Dragon Slaying!
A Chart of the various Slayers can be found here: