Rot grubs are nauseating parasites that feed on flesh and use corpses as nests in which to grow. While a rot grub can derive nourishment from dead flesh, its true hunger is for the flesh of the living—live flesh greatly accelerates a rot grub’s growth to adulthood. A grub with only a corpse to feed on can take weeks to complete its transformation into the short-lived, skittering, yellow-orange arachnid that is its adult form, but a grub that finds living prey gnaws its way through the body and gorges on live flesh, then nests in the resulting corpse and grows to adulthood in a matter of hours. An adult rot grub lives for only a few hours—long enough for it to lay dozens of eggs within the body of its old host before it dies. The new grubs that hatch grow slowly unless a new host wanders by.

Thankfully, rot grub swarms occur only rarely, as they require the infested carcass of a Huge or larger creature and many weeks to build up in the amounts necessary to constitute a swarm. When this occurs—after the grubs have finally skeletonized their host—they erupt and seethe across the ground, a hideous carpet of pale flesh that, unlike smaller groups of rot grubs, actively seeks out living flesh to consume. Once formed, a rot grub swarm is self-sustaining; the vermin quickly devour creatures they slay and move on. When individual grubs mature in a swarm, they are quickly eaten by those grubs yet to achieve adulthood.

Swarms of rot grubs are single-minded in their purpose, and know only hunger. They are voracious, and never stop moving or pursuing flesh that can be eaten. As long as consumable flesh exists, they replenish their numbers at a speed such that they never need to stop to rest. Swarms of rot grubs have been known to cross vast expanses of land like flesh-eating floods, consuming all living creatures that they happen to encounter. These menaces arise most frequently in warm, moist environs, but prove tenacious and might be found in nearly any unfrozen clime.

Infested Undead

Since rot grubs only feed on living flesh and use dead flesh as an incubator, undead are not harmed by infestations of the parasites. A corporeal, non-skeletal undead infested with rot grubs transfers one rot grub to any creature that touches it or to any creature it strikes with a touch attack or with any natural attack. The target may make a DC 15 Reflex save to avoid being infested, but otherwise is immediately subjected to a single grub Burrowing through his flesh. A rot-grub-infested undead’s CR increases by +1.