The Infested Ruins is a Lost Quest Map. It features wasps, a flying enemy exclusive to this map.
This map is one of the best locations in the game to farm high-end armor through Nightmare Hardcore Survival (if the player(s) can handle it).
During the final wave on campaign mode, the Wasp Queen boss will spawn in the northeast corner of the map. She summons wasps to aid her.
*These weapons can slowdown enemies on-hit:
Wasp: Reach Wave 25 of Survival Mode on any difficulty.
Serpent: Reach Wave 30 of Survival Mode on any difficulty.
Wasp Queen: Complete the final wave of Survival Mode (Wave 35) on any difficulty.