A type of species found in almost every Zone of the The Seven Realms, some regions more, other regions not at all. They are several sub species of Abyssa too. Cora belong to this species too.
Abyssa are mobs who will always stay at the same spot. When you are 5 levels higher or lower, they will not attack you anymore.
In Terratria's Reef, some Abyssa are located around F4, F5 and F6.
A summary of all the Abyssa in the game: Category:Abyssa