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