Emmerololth was an Ascian who learned about the primal Eureka imprisoned on the Isle of Val. When Emmerololth sought Eureka to use its power, Galuf Baldesion and his compatriots conjured a powerful spell to cast the entire island into the Lifestream. There, they used their combined powers to destroy Emmerololth, scattering her essence within the aether such that she would never again be able to reform.
Despite this, Emmerololth is present in a meeting of Ascians that takes place after the Isle of Val was sent into the Lifestream. Presumably this was a different sundered shard of the Ancient Emmerololth's soul that was ascended to Ascian status.
Like the other Ascian overlords in the reunion scene, Emmerololth is represented with a lesser ascian model. He wears a single black leather hooded coat with purple markings and adornments, and is wearing a black mask.
Behind the scenes
Emmerololth was one of the Ascians who were present at the reunion in patch 2.3 Defenders of Eorzea. This conflicts with the revelation in Eureka, the Forbidden Land that they were destroyed in the Isle of Val's disappearance, which canonically takes place before the scene in question. Revelations by fellow Ascian Emet-Selch may explain that this Emmerololth could be a replacement for the one destroyed by Galuf, however the truth of the matter remains unclear.