Kill the fish and swim through the hole down, kill another rot fish, and swim up to the gate, it will lower, grab the thunderbolt and jump out of the water, kill the scrag and collect all the goodies. Jump through the portal. A spiky floating ball moves away, collect all the weapons and follow it, eventually you'll see a shambler to the right. Kill the shambler and the wall ahead will open. Kill the shambler there, and proceed, try to avoid the nail traps and don't waste too much time tackling all the monsters, they will keep materialising. At the end of the corridor is a portal, to access the only secret are drop down of of the edge of the platform to a ledge below, and follow the it to a room with quad. Exit through the portal. To finish the game you must kill Shub-Niggurath, this can only be done if you telefrag it. If you step through the portal you will reappear wherever the spiky floating ball is, wait for the ball to enter the monster then enter the portal. You will telefag her and complete the game.