Based in Farron Keep, the Abyss Watchers are a group of Undead warriors who acted in the dark, seeking out any sign of the Abyss, fighting a constant war with its abominations. They "vowed to partake of wolf blood". Slightly ironic, or possibly on purpose, as Artorias the Abysswalker's companion was Great Grey Wolf Sif.
Being of Farron, they were lonesome warriors proficient with the sword, and relied only on more pragmatic spells. It is unknown whether one or all of the Abyss Watchers linked the fire. Perhaps they worked as a team to push back the darkness. But, they were betrayed by the Fire. The wolf blood that bound them to one another dried up, and Farron "was consumed by a festering wood".
The very power the Abyss Watchers drew from to combat the Abyss evaporated in the fire. Their home grew acrid, and the allies they left behind were warped into inhuman creatures. Worse yet, the Abyss was left unchecked, and now Darkwraiths roam freely in Farron, an affront to everything the Abyss Watchers stood for. When the Abyss Watchers awoke and saw what had become of their home, they were overcome with anger so intense it was maddening, or perhaps the encroaching darkness took control of them. Now the watchers sit on the doorstep of the darkness they once sought to extinguish present in the Catacombs of Carthus.