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