A Poison Swamp-related lab that makes basic enemies that are hurt by Poison Swamp take increased damage, increased by a percent of currently applied rend.
To have this apply to other enemy types as well, you will need the lab Swamp Rend Additional Enemies.
You unlock this lab in Milestones by reaching Tier 16 wave 50.
Swamp Rend - Basic Enemies - Lab values, costs, and duration (30 levels)