When large piles of bones are left in areas suffused with necrotic energy, such as in a desecrated ossuary, there is a chance the bones may rise and become reanimated on their own, forming into an animate pile of bones and skulls, a swarm of bones.

Swarms of bones act similarly to normal skeletons. They are obedient to their controllers when magically commanded, and they enjoy hurting living things when they have the chance to make their own decisions. A swarm of bones has considerably less intelligence than a proper skeleton, however, so most necromancers will rarely use these swarms in lieu of a cadre of skeletons. Typically a swarm of bones has no controller, and simply lurks in the area of its creation, waiting for living things to wander by and incur the swarm's wrath.

Swarms of beasts are dangerous enough, but a swarm made of undead can be even more difficult to disperse. Undead can be commanded to swarm, but they usually form swarms as an accidental result of necrotic energy lingering over many scattered remains.

Turning Swarms. Swarms of undead have a special relationship with effects that turn undead. Driving away the undead can cause them to scatter, disturbing the cohesion of the swarm just as damaging the individuals would do. Clerics that turn undead may find the entire swarm routed just as quickly as it appeared, assuming the swarm is made of undead that can be turned.

Undead Nature. These swarms are made of tiny undead that don't require air, food, drink, or sleep.