Residue Rendezvous is another one of the new weekly ICC quests. If this is your weekly quest, you will find the quest giver in the upper reaches, just past the teleport. The description of the quest is a little misleading. Although it specifically mentions Professor Putricide, he isn’t involved in the quest. Instead, you must fight Festergut and Rotface.
To do this quest, you must get a special debuff from both bosses. You get Festergut’s debuff by having a spore explode on you, which is a normal part of the fight. You get Rotace’s debuff by taking a tick of slime spray damage. Each debuff lasts 30 minutes and will be removed if you die or get DI’d. To complete the quest, only one member of your raid actually needs to have both debuffs (but I’d suggest having as many people as possible get them to be safe). Just have everyone run back to the quest giver and the whole raid will get credit for it.
It’s a good idea to clear all the trash in the hallway between Festergut and Rotface’s rooms before you engage one of them to avoid wasting time and the possibility of accidental deaths. If one of the bosses is more troublesome for your raid group, make sure to tackle that one first, as wiping on the 2nd boss will make you fail your quest.
Of course, there are ways to cheat and complete this quest without actually killing the bosses, but what fun is that?