How To Complete The Elden Ring Dung Eater Questline
Considering the sheer amount of wild, vicious, and downright bloodthirsty characters that players can encounter while playing "Elden Ring," it's hilarious that perhaps the most evil being in the entire game is named "Dung Eater." Dung Eater is an NPC that appears at Roundtable Hold sometime after you've acquired your first Great Rune, though it's unclear what exactly triggers his arrival. You'll know he's there once Roderika the Spirit Tuner warns you about a malevolent presence on the other side of the Roundtable. He appears as a red phantom invader, though he will be non-hostile and simply sit on the floor festering in his own filth when you first interact with him.
There is a lengthy questline attached to him, which actually allows players to activate a new ending to the game: one which causes the Lands Between to be overrun by a loathsome curse, courtesy of Dung Eater. To start his quest, players must first give Dung Eater a "Seedbed Curse," which can be found in various locations throughout the game, though the earliest one is in Leyndall. Start at the East Capital Rampart Grace and run down the ramparts until you reach an elevator. The Seedbed Curse is located at the very top of the first building you enter once you step off that elevator, on a corpse tied to a chair. Bring it back to Dung Eater, and you've officially started his quest.
Freeing the Dung Eater, and battling him beside the moat
After giving him the item, Dung Eater will give you a key to his cell in the Sewers beneath Leyndell. You can reach the sewers by jumping down a well near the Avenue Balcony Grace, and making your way into the Subterranean Shunning Grounds area. Fall down a grate next to the Underground Roadside Grace, and follow the path past several poison spewing flowers until you reach a ladder. Climb up the ladder to find the cell that Dung Eater is stuck in. Open it and exhaust all of his dialogue, until you reach the point where you can tell him to leave his cell.
After telling him to leave, return to the Roundtable Hold to find the message he has left for you in his usual room. The quest gets a little tricky at this point, as the next step is to go down to the Outer Moat area of Leyndall and fight the Dung Eater as an invader, but it seems like this portion of the quest may not activate unless you speak with another NPC: Blackguard Boggart (via Sipder).
Boggart is a thief who sells prawns, and has his own separate questline in Liurnia. It's reported (though not confirmed) that players will have to finish the first part of his quest to get him to move to the moat, or else Dung Eater will not invade. In any case, defeat the Dung Eater invader to progress to the next part of the quest, and loot the Seedbed Curse off of Boggart's corpse for good measure.
Locating the remaining Seedbed Curses
Return to Roundtable Hold and speak to Dung Eater, who will now tell you to bring his other body (the one in the sewers) more Seedbed Curses. Another Seedbed Curse is found in Leyndell on the 2nd Floor of the Fortified Manor (which is a recreation of Roundtable Hold) in the room that normally houses Dung Eater in the actual Roundtable Hold. The next Curse is in the Volcano Manor, and can be found after you reach the Grace just outside of Rykard's boss fight. Turn around and follow the stairs up to find it, though be warned that you'll need a Stonesword Key to access this one.
The last 2 can both be found in Elphael, Brace of the Haligtree, and Both can be found near the Prayer Room Grace. The first is located at the top of a rampart that sits at the edge of the area with a gazebo and scarab. The second is found by jumping down to the right of the small chamber with a Cleanrot Knight inside, and following the platform below until you're standing above a group of crystal warriors. If you've looted the Curse off of Boggart's corpse, you'll only need one of these last two Curses to complete the quest (via Gamerpillar).
Return and present the rest of the Curses to Dung Eater to officially finish his quest and receive the "Mending Rune of the Fell Curse," along with the Omen Set.