The Nine Hells
This symbol was first discovered in the Underdark by our heroes. Lore known to them suggests it's meaning was of a plane or group of planes known as, "The Nine Hells."
The Nine Hells, also known as Baator in Infernal is the home of the devils. It’s a plane of sinister evil and institutional cruelty organized in a caste system with a rigid chain of command. The devils are highly organized in their quest for power and status, scheming and plotting coups and assassinations. Each of the nine Hells has its own physical laws or properties of matter, but all are deadly to outsiders.