Void Monster adds a creature to the void which will pursue you for as long as it lives. It is trapped beneath the bedrock, so it is not a direct threat, but you will not be rid of its growls, grunts, and snarls nor the evil ambiance that surrounds it. This adds a whole new layer of ominousness and foreboding to the deep caves, and makes mining much more unsettling.
Should you find a hole in the bedrock, or be in a dimension such as the deep dark, which has natural holes into the void, the monster is free to enter your world and attack.
The monster is a dangerous foe; it will deal significant amounts of damage - two or three hits will kill an unarmored player - and if hurt, it will periodically heal itself, during which it is immune to attack. If you do attack it, it will charge you, usually hitting you again. Each time it hits you, it will heal itself one half of a heart, and will deal 10s of blindness. Its nether counterpart will also light you on fire. To give itself an edge in battle, the monster will eat many nearby light sources, causing them to drop as blocks.
Killing the monster will start a timer to spawn a new one in the void, and will provide a number of useful drops.
The mod contains a config that allows you to blacklist dimensions from the spawn rules. This is intended to stop spawns in void-type MystCraft ages, and will not have any effect if you blacklist the Overworld, the Nether, or Extra Utilities' Deep Dark.