Kangaxx's torso?

This is going to contain some spoilers:
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You probably didn't kill Kangaxx, you killed one of the two liches guarding his body parts. I don't remember which one guards the torso and which one the arms and legs, so I'll deal with both. You need to pick up the other parts. If you got the torso from the sarcophagus of the lich under the sewers, you need to go to the Bridge District, in the Southwest corner to pick up the other parts (behind a trapped door, down some steps and manipulate a sarcophagus, kill that lich and manipulate the sarcophagus again). If you got the torso from the Bridge District, you need to go to the Temple District, down one of the three or four sewer openings, through the northwest passage (you must have accepted the unseeing eye quest in this district or you can't go that way). You need to get through a locked door, but play along with the unseeing eye quest and you'll get there. Do the same thing I set forth above with that sarcophagus.

Armed (pardon the pun) with the arms, legs and torso, go to the Docks District. At the South end of the screen, just off and east of the stairs to the west side of the screen, pick the lock on the door of the first house, go in, go down the stairs, manipulate that sarcophagus, and play along.

If you want spoilers for how to beat Kangaxx, do a search on this site, it's been asked roughly 6,998 times.