A civilization of sufficient power and technological capability can eventually outgrow concerns such as being invaded or overthrown. These mighty empires instead eventually crumble from their own hubris. Once they reach too far and attempt to tap info fundamental forces of nature, to split apart the atom and gobble up the boundless energies inside, then do they truly reap what they sow. The consequences of these efforts are often all that survive these ruined societies, persisting as mere echoes of a failed world.
Echoes are unintended byproducts of efforts to harness atomic energy as a power source, or to unleash that energy all at once for weapons of awesome destructive power. The echoes persist as a miasma of noxious invisible particles which eat away at anything they encounter, including living beings. Creatures that breathe in or come in contact with these clouds are subject to a rapid decaying sickness known as radiation poisoning, which shuts down internal organs as the body is systematically destroyed on a cellular level.
Because echoes interact with matter at an atomic level, they are nearly impossible to dispose of and very difficult to contain, often requiring sealing away and burying deep beneath a world's surface in lead-lined tombs with signs displayed prominently warning others of the creator's folly. Some clever mages have found banishment effective, but sending one's problem to another plane of existence only transplants the issue without truly addressing it, and risks creating enemies of whoever is on the receiving end.
The only way to truly avoid echoes is to not create them in the first place, but the carrot of energy potential dangles enticingly in front of many a civilization too enthralled to sense the dangers therein.
Self-Sustaining. Echoes of a failed world do not require air, food, drink, or sleep.
Mindless. Echoes of a failed world do not possess a mind and cannot be targeted by any effects that require a mind or thought.
Incorporeal. The swarm can move through other creatures and objects normally and vice versa, unless the creature or object is completely encased in or made of lead. The swarm can move freely in a vacuum, and it can occupy multiple contiguous spaces of any shape. The swarm is invisible.
Swarm. The swarm can occupy other creature's spaces and vice versa, and it can move through any opening without squeezing. The swarm can't regain hit points or gain temporary hit points except through its Decay action.
Actions
Decay. The swarm bombards any creatures and objects occupying its spaces with radiation. Constructs, undead, magic items, and creatures or objects completely encased in or made of lead are immune to this effect. Targets must make a DC 22 Constitution saving throw. On a failed save, a target takes 1d100 necrotic damage, and on a successful save the damage is halved. If the target is a creature and takes any damage from this action, it is diseased, its hit point maximum is reduced by an amount equal to the damage taken, and the swarm regains the same amount of hit points. If this effect reduces the target's hit point maximum to 0, the target dies and its body breaks down to become part of the swarm.
While a creature is diseased in this manner, the reduction to its hit point maximum cannot be removed. The disease can only be removed by the wish spell or intervention from a deity. Once the disease is removed, a creature's hit point maximum returns to normal after a long rest.
The damage dealt is halved if the swarm has half of its hit points or fewer.
[id: monster stat block for echoes of a failed world, depicting a party of adventurers in silhouette, staring up at a sun in the shape of the international radiation symbol (trefoil) casting harsh white light upon a spike field laid out as a long-time nuclear waste warning message]
