Hyassalmo was built in the Horgon Era by drow colonists out of Eldalweril. In the Eldalweril Civil War, Hyassalmo ceded from the Eldalweril empire. Five years after this war, in the Year 23, and after seeing the economic dominance and quick recovery Hyassalmo was made the new capital of Eldalweril. An interesting twist in this change from the old capital of Jhaldrym (which had held that title for more than a millennia) to the new capital Hyassalmo, is that it came about from the urgings of a drow house in Hyassalmo which before the war was low and practically unheard of, yet by the end of the war house Agrach Dyrr was the most powerful across all of Eldalweril's holdings.
In the First Epoch, Hyassalmo was the main sponsor of raids up Eáránë Aldaríon and into the surface lands of Taurelin. In the Second Epoch, the drow of Eldalweril waged a great war with Angrod's surface elves. The conflict became known as the High Down War. By war's end Eldalweril was shattered. The capital city Hyassalmo was ravaged and de-populated by Angrod's military. The citizenry could not come back because sentient vines, the Tendrils of Taurquion, had squeezed their way through walls and streets. These surface vines ended up dying as nourishment was cut off to them and when they did they released noxious fumes and poisonous gas. As they decayed they became havens for deadly spores. Some of these budded into fungal lifeforms with sentience. As a result of these various menaces, Hyassalmo has never been re-occupied by the drow.