Here is the full list of all 24 Nihilumbra achievements.
Try the secret language.
Feed a Skeltogoth with a box.
Save the City in Void mode.
Beat the Void mode.
Save the Volcano in Void mode.
Clear level Volcano IV without killing any enemy.
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.
Save the Frozen Mountains in Void mode.
Smash two Crawlaggars with a box in less than one second.
Resist being absorbed by a Vaccumore during 10 seconds.
Kill a Klimeth without shooting it.
Die 10 times in a row playing Void mode.
Condemn the City.
Beat the Story mode.
Condemn the Volcano.
Find all the colors.
Burn four enemies in five seconds.
Condemn the Ash Desert.
Condemn the Living Forest.
Condemn the Frozen Mountains.
Escape from the Void.