The Erathi were a mysterious race of powerful magical beings dedicated to the god Mitharu, Lord of Order. They ruled the ancient Empire of Erathell. The Erathi arrived in Amalur after The Deep Gloam and this event began the Emergence era.
The Erathi built their empire and held it for over 400 years. Then they mysteriously disappeared and were replaced by the Mitharans, mortal followers of the Erathi who attempted (unsuccessfully) to hold the Empire of Erathell together after the mysterious departure of their immortal masters.
Very little is know about this powerful race and some during the Age of Arcana say that they were fictional.
The Erathi held powerful magic and created many complex fortifications during their time. It is unclear whether they meant to build the structures mostly underground or if they simply sank due to natural erosion. They also created the most sophisticated doors in the history of Amalur.
A strange note about the Erathi is that they seem to have an affinity for mysterious sea creatures. There are aquariums in some of the old ruins holding giant eel-like creatures. Whether the eels were pets, for studying, or another reason is unknown. Some of these aquariums were built overhead.