After killing the last of the ritualists Hellmaw will path one time around the room. He will stop where he initially was standing before the last mob was killed, then patrol up and down the northern most hallway indefinitely. It is best to pull the final ritualist far back to the southern entrance into the room. This will give you all the time you need to prepare for the final boss fight. You can also tank him in the tunnel leading to the room, so healers will not be feared out of the line of sight of the tank.