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