Hrm... how many pieces of the guy is there? I found two (bridge and temple districts), and to judge by the discussion here, that should do it. What then? How do I assemble them?

spoiler:

the head is in the docks district beside a tavern in the lower-middle area... (in short look for the house with the trap door and inside the house, there will be minatours guarding the place) remember though, he is one of the thoughest players in the game if you dont know how to handle him... just ask ard if you cant defeat him
Tip: (protection from undead helps alot against liches and demi-liches)