You've proven yourself as a competent warlock, adventurer. However, competence might not be all you need for the next challenge. Long now have those cursed Burning Blade cultists been practicing dark arts in Shadowbreak Ravine. They are able to summon powerful demons, but their brand of summoning is often deadly to the summoner. Go to Desolace and take part in one of their rituals. Ensure that you are well warded against shadow and have a demon servant of your own ready to defend you.