Dark Servants

Lore
The "Dark Servants" (term found in various in-game descriptions) are a heterogenous ensemble composed by entities spawned by the Abyss itself, creatures corrupted by it or beings that are bent to extinguish the First Flame in its name. A trait shared by this group is the weakness -and sometimes fear- for the energy of fire and the essence of the Gods: this weakness is the same of the Undeads because they too are linked to the Abyss, not physically but through the Dark Soul itself, while the Gods born from the First Flame. Non-human entities like lesser monsters and Gods can be corrupted by the exposition to the Abyss but their body will not be radically altered; human beings, possessing a shard (aptly called Humanity) of the Dark Soul, instead will become horribly warped: subjects of a rogue Humanity degrade into beast-like monsters, usually developing some sort of feeding maw in unusual places, multiple glowing red eyes and abilities centered around the Dark element and the draining of the enemies' souls.

Dark Souls
The lost city of Oolacile is the nest of the Bloatheads, the first example of living humans touched by the Abyss. A similar but even more atrocious transformation is what happened to Manus, twisted into an eldritch and misshapen behemot that did not even resemble a human being anymore. A similar fate is what happened to the Four Kings (once proud monarchs, reduced to soulless shells) and to Artorias, one of the Four Knights of Gwyn: belonging to the race of the Gods saved his physical form but still the Abyss shattered his mind and corrupted his body, which continuously seeped a purple ooze (the "corrosive darkness of the Abyss") that tainted his sword. The Darkwraiths are somewhat of a separate group because they are Undeads who willingly pledged their loyality to Darkstalker Kaathe, one of the most ancient emissary of the Abyss: they are literally the Crusader of the Abyss, armed with weapons made to harm the Gods and to steal Humanity.