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