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Max drowning damage capped at 40+2d10% of max HP.

3 max HP, 2 rings of regeneration.

Survive indefinitely underwater?

I also wonder whether there's some way to reverse those stairs (throw a potion of exchange downward?)

Trying to think outside the box. I have to feel like the scroll of omnipotence can't be read...

Dip it in a potion of raw chaos?

Argh, knew there was something. Feel free to kill the thread.

Alternately, who else could we give the scroll to? Is Khelavaster or the Ratling available on the return journey, perhaps?

Ok, so I actually play this game very little, and in fact I spend more time reading the wiki than playing the game, so if I miss some detail please forgive me.