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