| Hit Dice: | 12d12 plus 12 (90 hp) |
| Initiative: | +4 |
| Speed: | Fly 30 ft. (good) (6 squares) |
| Armor Class: | 20 (+2 size, +4 Dex, +4 deflection), touch 20, flat-footed 16 |
| Base Attack/Grapple: | +6/— |
| Attack: | Swarm (1d6 Str) |
| Full Attack: | Swarm (1d6 Str) |
| Space/Reach: | 10 ft./0 ft. |
| Special Attacks: | Distraction, Strength damage |
| Special Qualities: | Darkvision 60 ft., half damage from slashing and piercing, incorporeal traits, swarm traits, undead traits |
| Saves: | Fort +4, Ref +8, Will +8 |
| Abilities: | Str —, Dex 18, Con —, Int 2, Wis 11, Cha 18 |
| Skills: | Listen +9, Spot +10 |
| Feats: | Alertness, Toughness (4) |
| Environment: | Ethereal Plane |
| Organization: | Solitary |
| Challenge Rating: | 5 |
| Treasure: | None |
| Alignment: | Usually chaotic evil |
| Advancement: | — ♊ |
| Level Adjustment: | — |
A ghostly swarm of rats floats silently and eerily along the ground, creeping closer.
Ephemeral swarms are the ghostly collections of many little creatures that suffered a common death. Just as when a spirit of a particular creature lingers on as a ghost, when many small creatures die a violent death, they may linger on as a vengeful ephemeral swarm. The undead swarm is composed of the psychic agony and anguish of the newly departed.
The physical appearance of the creatures in the swarm corresponds to what was killed. Thus, in an area where a plague killed all the cats, the ephemeral swarm will look like a pack of cats and kittens. If a fireball spell took out a rats’ nest, the ephemeral swarm would seem to be a ghostly rat swarm.
Almost all ephemeral swarms return from the dead seeking to inflict pain and suffering similar to that which they experienced upon their deaths. If the agent of their destruction is visible and recognizable by the creatures, a swarm will direct its efforts toward killing him. Otherwise, a swarm surrounds and attacks any living prey it encounters.
Strength Damage (Su): The swarm attack of an ephemeral swarm deals 1d6 points of Strength damage to any living creature whose space it occupies at the end of its move. A creature reduced to Strength 0 by an ephemeral swarm dies. This is a negative energy effect.
Ephemeral swarms sometimes manifest in cities recovering from a terrible animal or vermin infestation. These undead swarms are the remnants of one or more swarms that were previously exterminated. Ephemeral swarms are also encountered in the ruined strongholds of the Dhakaani Empire and the haunted halls of Nolunthrone, a ruined dwarf city beneath the Ironroot Mountains in the Mror Holds.
Ephemeral swarms haunt the Underdark and various dungeons scattered across Faerûn (see Known Dungeons of Faerûn, pages 293–296 of the Forgotten Realms Campaign Setting, for likely locations). They also haunt the labyrinthine sewers of Faerûn’s oldest cities, the Mulhorandi capital of Skuld in particular. Some ephemeral cat swarms found under Skuld are particularly ancient and deal other types of ability damage in addition to Strength damage.
Source: Monster Manual III, page 50.