Here is the full list of all 24 Nihilumbra achievements.
Feed a Skeltogoth with a box.
Clear level Volcano IV without killing any enemy.
Try the secret language.
Save the City in Void mode.
Beat the Void mode.
Kill a Klimeth without shooting it.
Save the Volcano in Void mode.
Save the Ash Desert in Void mode.
Smash two Crawlaggars with a box in less than one second.
Save the Living Forest in Void mode.
Save the Frozen Mountains in Void mode.
Resist being absorbed by a Vaccumore during 10 seconds.
Die 10 times in a row playing Void mode.
Go where no one has been.
Condemn the City.
Beat the Story mode.
Find all the colors.
Condemn the Volcano.
Burn four enemies in five seconds.
Condemn the Ash Desert.
Condemn the Living Forest.
Make a bullet bounce 20 times.
Condemn the Frozen Mountains.
Escape from the Void.