Coalescipede

Coalescipedes are found exclusively within the darkest areas of Shaded Citadel. These normally non-hostile creatures are afraid of light and will flee from it, including light produced or carried by something, for example Lantern Mice, Scavengers carrying lanterns, or a Slugcat carrying a lantern or producing light after eating a neuron. However, large numbers of the spiders can aggregate, making them less fearful of light and consequently more hostile. Small clumps are harmless, but medium sized groups can potentially be dangerous to the Slugcat and Lantern Mice if they become brave enough to withstand the light. Large aggregations of these creatures coalesce into a centipede-like creature which is brave enough to bypass large light sources much quicker, so avoiding swarms is advised.

Strategy
Coalescipedes can be broken up by throwing items at them (e.g. spears or rocks), killing a few of those directly hit and causing the rest to disperse back to individual, small spiders and thus reducing their hostility. The intense light generated from throwing a flashbang will kill all coalescipedes in a single room.

Trivia

 * Scavengers in the Shaded Citadel have lanterns with them. This appears to indicate that they are aware that coalescipedes fear bright light, or they're simply unable to see in the oppressive darkness of the citadel. Trading for these lanterns is the only way to acquire them, outside of stealing, murdering, or looting a corpse.
 * Even though Lantern Mice generate their own light, individual Lantern Mice are unable to fend off most groups of coalescipedes.
 * There seems to be an ambient noise when coalescipedes are present in a room that resembles a deep droning sound.