'Umr At-Tawil is an entity who stands beside the Ultimate Gate, meeting any who traverse the first Gate via the Silver Key. He appears as a heavily cloaked, vaguely humanoid figure, with no organs or features discernible beneath his garb.
In the Pathfinder multiverse, it's a Great Old One known as Tawil at'Umr, which is not a well-known entity. Which serves Yog-Sothoth, operating as the physical projection of its will and working toward its agenda across the dimensions. Tawil at'Umr is theorized on Golarion to being the side of effect of Yog-Sothoth's contact with the material plane. It's knowledge about reality is vast, including how one access the Dimension of Time, but one acquires such knowledge at a terrible cost.
When it comes to its appearance, mortals see a cloaked humanoid figure which is somewhat larger than the observer. It's true form under the cloak is that of a protoplasmic mass which can take varying shapes, some of which should not be capable of existing.