Urzhaleth was a powerful being from the Nascent Expanse that exploited the catastrophic breach of the Twilight Veil at the end of the Mythic Era. It possessed enough coherent identity and intention to act deliberately against Creation, distinguishing it from transient manifestations that could not sustain a continuing purpose after crossing the boundary.
The breach that admitted Urzhaleth resulted from a Huraz effort to reinforce the weakening Twilight Veil. Once the hidden fault ruptured, lesser beings and aberrant manifestations accompanied or gathered around Urzhaleth, turning the crisis into an organized incursion. The greatest danger was not merely Urzhaleth’s individual power but the possibility that it might establish a durable relationship with Kor Sirok and use that foothold to draw additional entities through the damaged barrier.
The gods intervened before the incursion could mature into another conflict on the scale of the Twilight War. Rather than relying upon the Twilight Veil alone, Naevra and the gods weakened the relationships between Kor Sirok’s physical universe and the wider planes of Creation in the event remembered as the Sundering. Surviving mortal histories do not establish Urzhaleth’s ultimate fate beyond the failure of its incursion, and its precise nature within the wider variety of beings originating in the Nascent Expanse remains uncertain.