03-03-2007, 01:13 PM
Wow, it sure has been a long time since I've been to the unholy altar in single player. You need to take Lazarus's staff and talk to Cain in town. He should then open a portal for you on level 15 by the pentagram. You take that portal to the unholy altar. Go Rambo on that sublevel, and a portal will appear down there, taking you back to 15, at which time you can proceed through the pentagram to 16.