Meaning "The Most Ancient and Prolonged of Life", it is also said as Tawil At-U'mr or Tawil-at'Umr. It is described as an avatar of Yog Sothoth in the story "Through the Gates of the Silver Key". He presides over the timeless halls beyond the Gate of the Silver Key and the strange Ancient Ones that dwell there. He is described as the silhouette of a man behind a strange, shimmering veil. He is one of very few benign Lovecraftian Great Old Ones who does not cause insanity in those who view him.
The guardian of the Ultimate Gate that leads to Kadath Mandala gives his forgiveness for allowing the party and Lieutenant General Zula to enter, but had decided not to let them go. Zula explains that if someone does something behind his back he will not allow them to leave and those that face him in battle may suffer the unfortunate fate of being sent to an unknown dimension.
After Umr-At-Tawil's defeat he melts away. This reveals the Trapezohedron of Knowledge that belongs to Shiori Miyashiro. Zula hurries the party before Umr-At-Tawail regenerates more powerful than before. He is the boundary of the collective unconsciousness and consciousness separated for if he didn't there would be no self or individual entities. For this reason he cannot be killed permanently.