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