Early in his invasion of Aysle, Ultherion had a problem. Unlike Orrorsh, where Corruption was hidden by the reality, in Aysle is shone as darkly as Honor shone bright. His stelae became dark beacons for every hero of Honor in the land. A bargain with the Corrupt god Corba'al allowed the stelae's evil to be hidden, but the reality of Aysle would not be so easily thwarted: during the Entity hours, the twilight times between light and dark where Corba'al rules, the stelae show evidence of the dark pact between High Lord and Corrupt god. To protect his stelae during these hours, Ultherion was granted the use of a Nightmare, Cha'ken the Twilight Demon, which he then offered to Corba'al to seal their pact. Thus was born the Crepuscula.
The Crepuscula is truly a creature of Corba'al, existing only during the Entity hours, and even then not entirely on the material plane, remaining partially in the supernatural nothingness from which it was formed. Outside of the Entity hours, there is no earthly way of detecting a Crepuscula, and even Divination/Entity spells are of little use [+25 to the difficulty]. As twilight falls, a shadowy and vaguely humanoid - more often described as "bear like" - figure begins to coalesce from the approaching shadows. Though initially indistinct, the Law of Observation soon begins to act upon it, making it more and more "real" with each passing moment. A quarter of the way through twilight it has the consistancy of smoke, and can be heard as the faint rustling of the wind. Halfway it flows like water, and can begin to be tracked by earthly means. In the final moments of twilight, before it is banished once again to the supernatural, it is fully formed, and it's Orrorshan heritage is fully visible: overly long and muscular arms bend at impossible angles, multiple jaws are filled with row upon row of shark-like teeth, and numerous tentacles propel it equally well on land and sea, while huge bat-wings grant it some mobility in the air. It is also at this time that any supernatural protection the Crepuscula had is gone, and it is at it's most vulnerable.
It is because of this eventual vulnerability that Ultherion no longer depends on the Crepuscula to defend his stelae. Also, having conquored Aysle and corrupted the Law of Corruption to allow evil to be hidden similar to Ororrsh, his stelae are no longer as obvious as before. But the Crepuscula remains a part of the thaumaturgical ecosystem of Aysle, sometimes used by the god Corba'al to his own ends, sometimes found roaming free, and occasionally even guarding stelae, drawn to their old service by pure instinct.