He spawns in the Ferumbras tower, Of which you need to get access to from standing on different points in the warlock re-spawn. Ferumbras is known for his demon summons and minions. One of the hardest boss's known to tibia suggestion is to take a big team, kill the demons and then secure in Ferumbras into a corner in order to damage him down. Warning when entering into the room you are in the middle, Portal is to the east if you wish to try and save your life!
You also receive Ferumbras necklace from entering the room and completing the quest. Note- Ferumbras does not need to be spawned in order to get the neck.