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