Good grief! You get through the door easily and into the inverted ziggaraut
aquarium. To get to the door at the bottom you must defeat the guardian monster.
It's hard to place, but it looks like an Amphibious Titanic Paragon Fiendish Dire Half-Scorpion/Manticore!
A) Hack n Slash.
A) Use spell arsenal.
A) Tumble past it.