Dung Eater is an Invader Introduced in Elden Ring. You don't need to defeat him to complete the game, but it's necessary to kill him to complete 
Milos (Dung Eater)'s questline.
I'll defile you next. Come to the outer moat.
Dung Eater Main Information
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Dung Eater is the invader version of
Milos (Dung Eater)
.
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Drops the
Sword of Milos
.
Where To Find the Dung Eater
Dung Eater will invade you mid-way through his questline or at the end of 
Blackguard Big Boggart's questline.
Dung Eater will appear for the first time at the 
Roundtable Hold as soon as you get access to the 
Altus Plateau. He's inside the room past the 
Twin Maiden Husks. Talking to him right away won't do much because you must first find at least one 
Seedbed Curse. It's recommended to locate them all before continuing with the Questline.
As you get a Seedbed Curse, head to the Roundtable Hold and show it to Dung Eater. He'll give you the 
Sewer-Gaol Key, a key that opens the zone where Dung Eater's real body is trapped.
The next step is to head to the sewers to find Dung Eater's real body. To do so, head to 
Leyndell, Royal Capital. Exactly, at the West Capital Rampart Site of Grace. From here, head northwest to get to the streets. It's a straight path from the Site of Grace to a ladder where you must drop next to. You'll see a small town at the bottom; get there. Now that you are there, try to locate a well at the northeast; it'll take you to the sewers.
Inside the sewers, explore until you officially reach the 
Subterranean Shunning-Grounds, drop to the bottom zone, and walk all the way to the northeast until you see an entrance to a lower zone. Midway through this path you can find the Underground Roadside Site of Grace.
As you find the entrance, drop down, and walk to the northwest and keep going through the main path. You'll eventually see Giant Poison Flowers and a ladder. As you reach the top, turn right, and you'll find Dung Eater, hitting his head with a wall.
Now you have two options; you can either kill Dung Eater here or you can continue the questline. If you kill him, you'll lose access to the Blessing of Despair ending, but you'll get the 
Omen Set and the Sword of Milos.
Ending the Questline
To continue the Questline, tell Dung Eater to leave the Gaol. Head to the Roundtable Hold and go to where Dung Eater was initially found. You'll find a message here; read it. It's a warning; if you head to the outer moat, you'll get invaded.
Now, go back to the Altus Plateau. From the Capital Rampart Site of Grace, head to the bridge and follow the road until you find a small lake. Dung Eater will invade you here. Here's when you can get the Sword of Milos.
