The Black Blood

In the deepest areas of the Underdark, where the borders between it and the Deep Dark are thinnest, a substance referred to by the Deep Gnomes as Black Blood has been spotted. This is a dark substance viscous substance that if touched by living matter will alter and mutate the creature touching it. The only creatures seemingly unaffected by it, at least visibly are undead and aberrations.
The substance has at times showed evidence of limited sentience like some forms of ooze. however may believe that it is mainly tied to something larger and darker trapped within the Deep Dark since the earliest days of the Dawn Era. The exact nature of the source is unknown, however due to the creatures immune to its effects, it has been speculated to have origins tied to the Far Realm. Definitely all can agree it is something alien to Khinarzi, the best hope is the origins remain unknown and its source locked away.