There are three different endings to Hollow Knight, each requiring a different set of circumstances.
Ending 1: The Hollow Knight
Requirements:
- Not having acquired Void Heart
Upon defeat, the Hollow Knight dies and The Knight absorbs the infection into themself. The Knight is then sealed inside the Black Egg and the game ends.
Unlocks the "The Hollow Knight" achievement.
Ending 2: Sealed Siblings
Requirements:
- Having acquired Void Heart
- Defeating The Hollow Knight
Near the end of the fight, Hornet will swoop in and fracture the Hollow Knight's mask with her needle before restraining it and giving The Knight an opening.
If the player strikes The Hollow Knight enough times while it is restrained, it breaks free, knocking Hornet unconscious, and the fight continues. This causes an ending where, in addition to the events of the first ending, Hornet is also sealed inside, her mask appearing on the door (similar to how the Dreamers' masks were before).
Unlocks the "Sealed Siblings" achievement.
Ending 3: Dream No More
Requirements:
- Having acquired Void Heart
- Defeating The Radiance
While Hornet has the Hollow Knight pinned down, if the player uses the Awakened Dream Nail on them, they will be transported inside Hollow Knight's mind to challenge The Radiance. At the end of this fight, the Hollow Knight returns as a Shade that rips open the Radiance's face, allowing the Knight to destroy it once and for all.
After The Radiance is defeated, the Infection is purged and the Black Egg remains open. Hornet wakes up to find the Knight's mask broken in half at the centre of a crater in the room, implying the Knight has either died or returned to the Void.
Unlocks the "Dream No More" achievement.
Extra Ending: Passing of the Age
After any other ending, if the player has completed Mister Mushroom's quest by talking to him in all areas, a cutscene will play after the main cutscene depicting Mister Mushroom flying off into the sky.
Unlocks the "Passing of the Age" achievement.
Trivia
- After any ending, loading the save file again will bring the Knight back to the Bench inside the Temple of the Black Egg, while recording the defeated Bosses in the Hunter's Journal.
- This means that if the player acquires the Void Heart prior to reaching the end of the game, they will no longer be able to unlock the first ending from that save file.
- Acquiring the Void Heart also unlocks the "Void" achievement.