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