How I Stopped Worrying and Learned to Love Eberron
These tentacled-headed creatures are often known pejoratively as “mind flayers.” But, to be fair, they have been known to flay a mind or two. Illithids usually live deep beneath the earth, in the tunnels and chambers of Khyber. Their true origin, according to texts from Dhakaani Empire ruins, is Xoriat, as minions of the daelkyr (they are also said to have assisted with the creation of the githzerai and githyanki races). Left behind after the Daelkyr Invasion, the illithids established vast underground feifdoms.
This psionic race starts life a cephalopod, crawling in and out of cave ponds and underground rivers. When they find a suitable host (usually an adult humanoid, which can include Umbragen, duergar, and any explorers caught down below), the infant illithids leap and attach themselves to the host’s head. After enveloping the victim’s skull, they begin to digest the flesh, and they use needle-like probes to invade the creature’s brain. Within a few days, the illithid’s skeletal structure has adapted and attached to the host’s spine, and the adult illithid is born.