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