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