A swamp nymph is a type of undead monster that appears in dNetHack. It is the most powerful type of nymph that most players will encounter randomly - stronger monsters in the class only appear to specific roles. Swamp nymphs are part-plant, and like many nymphs they are fey creatures.
Swamp nymphs cast spells in addition to stealing items, and sometimes retaliate by "collapsing into a noxious fluid" when badly wounded in melee. This kills the the swamp nymph immediately, but will inflict disease on the attacker.
At least one swamp nymph is guaranteed to appear in the lair of Zuggtmoy, and two swamp nymphs named Surrarr and Nueate appear within rooms on each side of Mammon's throne room in Minauros. In one variation each of the lairs of Malcanthet and Graz'zt, the random nymphs that can generate upon their creation are always swamp nymphs due to generation rules.
Swamp nymphs are the most difficult regular enemy in their monster class by far, especially if one manages to steal your source of magic resistance.
As a forest pathogen, a fungus can be very destructive. Its high destructiveness comes from the fact that, unlike most parasites, it doesn't need to moderate its growth in order to avoid killing its host, since it will continue to thrive on the dead material.