Umbral Black

Umbral Black is a nightmare from The Diamond who was an apprentice in the Arcane Cog Magic Item Seller but left abruptly during The Migration of Bloomwilt that led the Nightmares and Kalashtar to leave their cities and start migrating north. 

When Sigholdt Sofiasson spoke to the owner of the Arcane Cog, they gave Sigholdt a formal warning letter to give to Umbral Black.