Coalescipede

Spiders are found exclusively within the darkest areas of Shaded Citadel. These normally non-hostile creatures are afraid of light and will flee from the slugcat when carrying a light source. However, large numbers of spiders can aggregate, making them less fearful of light and consequently more hostile. Smaller clumps of spiders are typically not dangerous, but will kill and snatch lantern mice from slugcat. Medium sized clumps can potentially be dangerous to the slugcat. Large aggregations of spiders form what looks to be a "spider-pede", which will attack the slugcat irrespective of a light source.

Strategy
Spider clumps can be broken up by throwing items at them (e.g. spears or rocks), causing them to disperse back to individual, small spiders and thus reducing their hostility. The light generated from throwing a flashbang will typically kill all spiders in a single room.

Trivia

 * The number of spiders needed to kill you depends on their size: 3 when large, 6 to 7 when medium.
 * Scavengers in the Shaded Citadel have glowstones with them. This appears to indicate that they are aware that spiders fear bright light, or they're simply unable to see in the oppressive darkness of the citadel.
 * Even though lantern mice generate their own light, individual lantern mice are unable to fend off even small clumps of spiders.
 * There seems to be an ambient when spiders are present in a room that resembles a deep droning sound.