Fetid Rot comes from the Grimelord that spawns in phases 1 and 2. It is best for two players to bounce the debuff between each other, and then share it with the rest of the raid bar one when Helya is on low health.
Patches and hotfixes
- Hotfix (2016-12-06): "The Patient Zero achievement should now be obtainable, as intended."
- Patch 7.1.0 (2016-10-25): Added.