From Path of the Vampire Wiki
World bosses are a special breed of NPC which have an enormous amount of health persistent between NPC encounters, slowly decreasing with each successive battle. Most often, it is required for them to be taken down by a dedicated team. They spawn randomly, much the way Raids do.
The world bosses spawn in the city or the wilderness (usually the latter). Bar rumours will inform you when a new world boss has spawned, and then it's up to you to locate them.
The Ancient Tizheruk are half-fae creatures who appear to be human during normal weather. However, when it snows, their appearance morphs into a monster with a snake-like long neck, fins, a massive head with gigantic fangs, and a fish-like tail. They have power over certain elements of ice (the ancient ones have much more control over the elements) and can disappear into the snow or bodies of water. They heal almost instantly and are incredibly strong. It is unknown why this particular ancient enemy has come to Harper Rock, but it seems bent on hunting and killing vampires.
Nobody knows who made it, or how it got to be so huge and powerful, but an "alpha fadebeast" appeared loose in the city in early July of 2014. The fadebeast attacked vampires in their own lairs and stalked the sewers looking for easy prey.