Personally, as a custom warlock... I could see the following possibilities...

Evoking Hellfire... "Breath-weapons", Blasts, Rays, Orbs, Bolts Barriers and weapon-shaped manifestations of it.
Transmutation of Oneself... like strength, quickness, claws, wings, scales, elemental/poison resistances and faster healing/recovery.
Evoking Areas of Darkness... (coupled with cold perhaps) and have the ability to see through it.
Conjuration... creation of poisons and summoning (at your own risk).
Divination... such as using another's senses, scrying, object reading and aura detection.
Abjurations... such as circles of protection, shields, barriers, dispelling, protection of one's mind.
Enchantments... such as charms, despair, fear, commands, suggestions, geas/quests and domination.
Necromancy... with regards to inflicting life disruption, fatigue, exhaustion, weakening, wracking pain and curses.
Illusions... invisibility, disguises, images of monsters and other false sensory information