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