Yibb-Tstll is an entity Brian Lumley added to the Cthulhu Mythos, appearing as a gigantic, bat-winged humanoid wearing a green robe, its body studded with detached eyes.
The being is said to perceive all of time and space, and beneath its cloak shelter countless nightgaunts. It is described as a child of Nyarlathotep, and is noted to have a close connection to the Great Old One Bugg-Shash.