The layer appears to be composed of the innards of some impossibly huge creature. Veins flow like rivers through vast tunnels of organic matter. Fleshy organs the size of boulders hang like fruit from the distant ceiling, secreting corrosive fluid onto living fields soaked in natural acids. Nothing survives for long within Ulgurshek.
The Fraternity of Order catalogues Ulgurshek as a 'living layer" to account for its apparent sentience. In fact, the layer is an ancient entity that predates the Abyss and the Great Wheel itself. The godlike being had already fallen into torpor when the Outer Planes themselves took form. Ulgurshek wasn't captured by the Abyss - it enveloped him as his dormant form drifted through the raw creative matter of the multiverse. Confined by reality, Ulgurshek has no hope of escape.
Because its memory plumbs a time-lost era unknown even to the obyriths, Lolth occasionally ventures into Ulgurshek to consult it on some sagely matter, and perhaps probe ir for rales of the strategy of its ruthless now-lost brethren.