Guuthal

 was an ancient black dragon and a dracolich during the Age of Arcanum.

Appearance
All that is known of Guuthal's appearance is that it was a particularly ancient and mighty black dragon as its life was unnaturally extended.

In the Monster Manual, which may not be canon to Critical Role, black dragons' already skull-like heads take on an increasingly skeletal appearance as they age: the flesh around the cheekbones and horns breaks down as if eaten away by acid. Their scales also become thicker and less glossy with age.

Once a dragon becomes a dracolich, it becomes even more skeletal in appearances as its scales and skin draw tight to its bones or slough off. The eyes are replaced with floating points of light.

Biography
Vex'ahlia read in Opash's journal about this very ancient black dragon instilled with necromantic energy that terrorized the world in the Age of Arcanum. Through forbidden rituals and sacrificing its life force, it was born again as a "new form of unlife" called a dracolich.