Miasma

Miasma is a highly toxic substance that poisons, slows and gives a 50% chance of rotting one HP per turn. It is generated by death drakes, rotting devils, and a few uniques, as well as by evaporating potions of decay or casting Corpse Rot. Some maps have miasma cloud generators, including a particularly difficult Swamp ending.

Strategy

Poison resistance will protect you from the poison, but the other effects are more difficult to counter. An amulet of stasis will block the slowing, but this is inadvisable, given that one will likely want to teleport or blink out of a miasma cloud. The rotting can only be countered through rotting resistance, obtained by being undead, having the Foul Stench demonspawn mutation, being in statue Form, being in lich form, or by being a high-piety Zin worshiper. Mostly, it should be avoided: use controlled teleports if you get the dreaded undead swamp ending, and abjure or run away if a draconian caller summons death drakes. If you find yourself in miasma, get out of it as quickly as possible: blink, teleport, run away - whichever will get you out the fastest.