Crafters have a chance to make an elemental item while crafting, this chance is increased by the skill Arms Lore; however, the slayer chance from Arms Lore only applies if you are a Grandmaster.
These weapons are attuned to killing a specific foe. They do much more damage to that creature type than magical weapons. Creatures have enemies too! Slayer weapons will drop from creatures attuned against their enemy. For example; Undead creatures are enemies with humanoid creatures so they have a chance to drop Repond Slayer weapons while humanoid creatures have a chance to drop Undead Slaying/Silver weapons which are strong against undead. Each slayer type has an opposite enemy type that has a chance to drop them. If you know you’re going to be fighting a specific creature type for some time it would be wise to invest into a slayer weapon against that creature type.
Repond: Humanoid creatures (Bandits, Ratmen, Goblins)
Fey: Cu Sidhes, Reapers, Boglings
Abyss: Demonic Creatures (Imps, Demons)
Reptile: Scaled Creatures (Lizardmen, Dragons, Drakes, Wyrms, Snakes, Jukas)
Vermin: Rats, Larvas, Worms
Silver: Undead Creatures (Skeletons, Liches, Ghouls, Mummies, etc.)
Elemental: Elemental creatures (Water elemental, Fire Elemental, Shadow Elementals etc.)