Morrigan

Morrigan, the Dark Moon, Erregal in Draconic, is the goddess of rot, disease, and pestilence in both the warm-blooded and Draconic pantheons. She is associated with the darker of the two moons of Morn. Her brother, Silvanus, is associated with the light moon.

Warm-blooded Pantheon

In the warm-blooded pantheon, Morrigan is usually depicted as a feminine figure with withered robes and rotting skin. Where she walks, she leaves a trail of maggots and rotting vegetation.
At the time of Creation, Morrigan did not exist. Instead there was Mieli, the god of nature. After the death of Mortus, Mieli was ripped in two and became Morrigan and her brother Silvanus, represented by the dark and light moons respectively.
When Morrigan emerged, she used the new existence of death to create rot and disease. Her role was associated with what would happen to natural objects after they died. Morrigan is usually associated with the negative aspects of nature, however this is not an entirely accurate representation. As part of her role in the creation of rot, new organisms are created that feed on the remains of the dead. These new organisms also bring new life and are a necessary part of the natural cycle.
Morrigan is the god responsible for teaching mortals about necromancy. She did this against the wishes of Solene, the goddess of magic.

Draconic Pantheon

In the Draconic pantheon, Erregal is depicted as a dark green dragon. When her wings are extended, their membranes are shown to be withered and rotten.
Erregal was originally Meferi, the creator god of nature. During the fighting with the interloper gods, Meferi was split in two, creating Erregal and her brother Zagal.
Erregal taught dragons how to use magic, corrupting them with arcane knowledge. By learning about magic, those dragons abandoned their commitment to their own inner fires, instead relying on magic. As they abandoned their inner fires, their bodies withered and weakened until visible rot grew between their scales.
The movement of the two moons throughout the monthly cycle is thought to depict the constant battle between Zagal and Erregal. As the two moons move in predictable monthly patterns, their fight is cyclical, repeating in the same way every month, with neither moon winning. Erregal is thought to sometimes try to help Naga in her fight against Saule at night, however she is usually too busy with fighting her brother.