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.
Save the Living Forest in Void mode.
Feed a Skeltogoth with a box.
Smash two Crawlaggars with a box in less than one second.
Kill a Klimeth without shooting it.
Go where no one has been.
Save the Frozen Mountains in Void mode.
Resist being absorbed by a Vaccumore during 10 seconds.
Clear level Volcano IV without killing any enemy.
Make a bullet bounce 20 times.
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.
Condemn the Living Forest.
Condemn the Frozen Mountains.
Escape from the Void.