Slayer is an elite melee military dwarf obtained by upgrading a Berserker dwarf. The slayer sacrifices armor for a very high damage output.
It is a final form military dwarf.
The Slayer is a reference to the Dwarven Slayers of Warhammer Fantasy fame, typically seen as bare chested dwarven warriors with a mohawk and with a weapon in each hand who have dyed their hair bright orange.