Midzyas/Way of the Nail

The Void Hunter is a hidden boss in Hollow Knight: Way of the Nail. He is essentially Midzyas' higher being form and the true final boss of the game. His immense power is shown when he atomizes the Nail Lord after entering his arena with the criteria completed.

Triggering the Fight
There are a few criteria for triggering the Void Hunter boss fight. All must be followed or entering his arena will not spawn him.
 * Scorpion and The Nail Lord must have been defeated first.
 * All four Void Beacons must be activated.
 * They are located in the City of Silk, Forests of Soul, The Light Grotto, and The Capital of Firecrest.
 * They will not spawn until the Nail Lord has been defeated.
 * Interacted with the boomerang left behind by Midzyas and have talked to Scorpion after interacting with the Boomerang.
 * All Nailmasters, whether they be bosses or NPCs, must have been interacted with.

Mechanics
When the arena is first entered, the arena will be locked and a second boss battle with the Nail Lord will trigger. After his first phase HP has been depleted and he performs one second phase attack, the battle will abruptly end as a dark, winged figure drops into the arena. It will then stare at the Nail Lord for a brief period of time. Afterward, he will dash behind the Nail Lord and stab him with one of his Void Lances. After being attacked, the Nail Lord will dissipate, utterly obliterated by the Void Hunter. Afterward, the Void Hunter will stab his tendrils into the ground, corrupting the arena. After the arena is fully corrupted, the Void Hunter will let out a terrifying roar, staggering both Red and Grisan before the battle begins. But before the battle truly begins Red and Grisan would enter sync mode. Sync mode would last for the entire fight.

The Void Hunter has several tricks up his sleeve during the battle, ranging from quick-time events to UI manipulation. He can make the UI look like Red or Grisan's HP is lower than it actually is, and sometimes pulling off fake attacks to trick the player further. His quick-time events, if failed will result in Red and Grisan being separated, and a menu will appear to select Red or Grisan. If one is not selected within 10 seconds, the game will randomly select the player. The chosen player will be teleported to a mini arena. Grisan will have to fight a void-corrupted Stitch, while Red will have to fight a void-corrupted Grimm King. At the end of the Stitch fight, Grisan will have to escape the arena while it collapses. Red will have this part skipped, but four unnamed mini bosses will spawn after defeating him.

Trivia

 * Normally, Midzyas would only attack if he was threatened first. It is unknown why the Void Hunter attacks on-sight.
 * It may have been due to some sort of corruption attacking Midzyas, but this is unconfirmed.