Here is the full list of all 16 Aeternum achievements.
Clear the Extra stage on the Demonic difficulty.
Defeat Winter on any difficulty.
Defeat Greytail the Wicked on any difficulty.
Defeat Samantha Clementine on any difficulty.
Defeat Millicent Elmsley Charles Helena Astor on any difficulty.
Defeat Candynce Sky on any difficulty.
Clear Story mode without continuing on the Demonic difficulty.
Clear Story mode without continuing on the Lunatic difficulty.
Clear the Extra stage on the Lunatic difficulty.
Clear the game on Story, Arcade, Arrange and Extra modes with Ellica on Demonic Difficulty or better.
Clear the game on Story, Arcade, Arrange and Extra modes with Aubrey on Demonic Difficulty or better.
Clear the game on Story, Arcade, Arrange and Extra modes with CeeBee on Demonic Difficulty or better.
Clear Story mode using any character without getting hit and without using a bomb on Normal or Hard Difficulty.
Clear Story mode using any character without getting hit and without using a bomb on Demonic Difficulty.
Clear Story mode using any character without getting hit and without using a bomb on Lunatic Difficulty.
Complete the tutorial.