A rhaumbusun, :, is a type of monster that appears in SLASH'EM, SlashTHEM and Hack'EM. It is an oviparous, carnivorous lizard with poison resistance and a damaging gaze attack that can paralyze you for 1-4 turns.
Rhaumbusuns are normally only generated through the summon nasties monster spell. They are technically eligible for random generation in Gehennom, but owing to their low difficulty, they will not naturally spawn there.
Individually, rhaumbusuns are not threatening: their paralysis is short-lived, and they lack the ability to inflict any physical damage. However, they serve to make the summon nasties spell far more dangerous than in vanilla, especially if you lack free action: a small handful of turns may be enough to seal your fate if you are immobilized while surrounded by summoned nasties and other high-level monsters. Fortunately, their low level, slow speed and high AC mean that they are very easy to kill once the paralysis can be nullified - reflection does not protect against the rhaumbusun's gaze attack, but blinding yourself will prevent it entirely, while free action will negate its effects.
Izzy, the rhaumbusun that resides in the black market, can prove a similar danger for players that either accidentally anger One-Eyed Sam or opt to actively fight him. Players without free action should watch carefully for summoned rhaumbusuns, and blind themselves as soon as they see one.
As a polyform
Doppelgangers in the early game can use the rhaumbusun polyform as a means to get an edge against much stronger monsters, since it is available as early as the second level. However, the form's weakness means they usually cannot take advantage of the gaze directly - instead, they can lay eggs to obtain tame rhaumbusuns that can use their gaze attack against suitable targets, making them much easier to eliminate.
The rhaumbusun first appears in SLASH 6.
The rhaumbusun is a monster from Dungeons & Dragons that first appears in the August 1981 issue of Dragon Magazine, featured in “The Dragon's Bestiary: Rhaumbusun” by Ed Greenwood and Victor Selby. A relative of the dreaded basilisk, the rhaumbusun is a reptilian creature with eight short and stubby legs, dorsal spikes and gemstone-like eyes. Rhaumbusun were clumsy and moved by loudly shuffling, making them easy to detect.
Their main weapon and defense mechanism is a paralyzing gaze that reached up to 20 feet and could affect creatures that met its stare for up to eight minutes; unlike many such gazes, this did not affect the Astral or Ethereal planes and could not be reflected with mirrors (though a spell of gaze reflection still works). A rhaumbusun would bite only when cornered, and usually targets creatures that their gaze has paralyzed.
Herbivorous and apprehensive by nature, rhaumbusuns often inhabit desert and steppe biomes, appearing in mated pairs and hiding their nests in small, dark and dry caves decorated with small shiny baubles. Their reputation as "predators" is a result of them paralyzing intruders that then become prey for actual predators in the area - some smaller creatures make their nests near rhaumbusun territory as a form of protection.