They are the embodiment of the Abyss, as Manus and Artorias both had black ooze emanating from them as well. The Pus of Man (parasite creatures) are also weak to the Wolf Knight Greatsword and Farron Greatsword, as you know, both weapons are linked to fighting the Abyss. I believe the Abyss is slowly corrupting the world of Lothric as the fire fades. Champion Gundyr never found his fire keeper and his body was corrupted by the Abyss, becoming Iudex Gundyr.
The Clerics were cognizant about this state of corruption.
So what about the Wyverns of Lothric Castle? There's not much information about them that can be gleaned from the lore. Why do they both die when one is killed, yet their Abyss creatures stay alive?
The whole concept of the Abyss was not fleshed out very well in this game as of yet. I think Pimmsy may just be dissapointed as you continue onward.