Here is the full list of all 10 Hippocampus: Dark Fantasy Adventure achievements.
Collect all the achievement
Complete chapter 1 - Trauma
Complete chapter 2 - Shapeshifter
Complete chapter 3 - Lorelei
Find all collectibles
Cut 50 limbs
Kill 4 enemies in a single shot
Find the portrait of Moebius' father
Do an execution with the whip
Destroy 25 vases