In Gehennom and on the Plane of Fire, the ground is hot enough to
boil potions (although not hot enough to, e.g., burn scrolls).
The potions sometimes survive this (almost always if it's your
potion, it's blessed, and you have maxed Luck), but often don't.
In addition to making a lot of flavour sense, this serves a
gameplay purpose in that it reduces the number of potions that
are deathdropped by monsters in the late game. In Gehennom,
monsters often generate with potions to use defensively, but then
get killed before they have a chance to use them: this produces a
surfeit of potions that players tend to convert into holy water or
potions of full healing (and in general it doesn't make much sense
that the basic potion of healing primarily generates in Gehennom).
This commit approximately halves the number of useful potion
deathdrops, whilst still allowing monsters access to their
potions; when the monster dies, it drops the potion and this has a
chance of destroying it.
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