The Warlock was employed by either Bison Dragonclaw or the Necromancers and possibly Roarhaven mages as they mention about sending them again in Kingdom of the Wicked as a whole to assassinate Skulduggery Pleasant and Valkyrie Cain as they chased Dragonclaw.
The Warlock died during his fight with Skulduggery and Valkyrie when his powers were bound as some sort of Warlock self-destruct mechanism. Just before he died, he warned Skulduggery and Valkyrie to "look out for us", as his brother and sisters would be after them.
Powers and abilities
The Warlock primarily attacked with beams of yellow light from his palm where it was described that the skin 'bubbled and boiled' then burst open to shoot a beam which paralyzed. He was also able to heal major wounds as shown when he healed a bullet wound in his neck. When he attempted to consume Valkyrie Cain's soul his mouth would widen and his teeth looked 'bigger, darker and stronger'.