Here is the full list of all 20 Eresys achievements.
Banish the ancient one
Complete the book of the order
Create 4 accursed blood spheres
Cleanse the shrine of Innsmouth
Get killed by the otherwordly
Succumb to the creatures of the sea
Summon the old one
Collect 25 lost artifacts
Banish the ancient one on a mad man mission
Summon the old one on a mad man mission
Witness the reveal of a diety
Witness the reveal of a diety, on mad man difficulty
Perish in the throne room of the abyssal temple
Retrieve all lost artifacts from the abyssal temple
Cleanse the shrine of Innsmouth, on mad man difficulty
Retrieve the lost seals from Innsmouth