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