What are the giant slimy snake things?
In Dark Souls 3, there is a recurring enemy (normally appearing in Hollows) that explodes forth from an enemy, in the form of a giant, tar-like snake-creature.
What are these enemies, and do they hold any significance in the world of Dark Souls 3? Why do they appear in such significant enemies as the Wyverns in Lothric Castle, and Iudex Gundyr, but scarcely anywhere else, save for the occasional Hollow?
TL;DR
They are probably dragon experiments by Oceiros and the Lothric Knights. They can also occur naturally by hollowing.
These enemies are called "Pus of Man" and unfortunately there is no official explanation from FromSoftware about what they really are (this applies to about 80% of things in Dark Souls).
There are however some things we can say for certain:
- They are creatures or mutations originating from the Abyss as can be seen by their red eyes, their weakness to fire and their resistance to dark.
- Since they are connected to the Abyss they are a form of impure humanity and can only occur on undead with the Dark Sigil.
- Impure humanity can transform the undead into beast-like creatures (The flies in the painted world were human once aswell as all the Tundra Knights and Vordt)
- They have some features of dragons and/or serpents.
- Lothric Knights were allowed to breed dragons (Source: Knights Ring)
Now the rest is mostly speculation (Watch this video for the full explanation)
They only occur around Lothric castle since they are dragon experiments by Oceiros and the Knights. (Note that the most Pus of Man are found in the Consumed King's garden)
The wyverns in the Dragon Barracks are already dead. They are just hosts for the Pus of Man. If you kill the infection the wyvern will disappear completely.
The one thing that doesn't fit 100% into the whole dragon experiment scheme is the infection of Gundyr. Although it might just be his impure humanity that transformed him into an abyss like serpent/dragon. He was hollowing because an unknown warrior defeated him while he was on the way to the Firelink Shrine. (The player is the unknown warrior and defeats Champion Gundyr in the age of dark)