Void Heart

Usefulness
Allows access to the second, third, fourth, and fifth endings of the game. Once the player has acquired the Void Heart, they cannot access the first ending without starting a new game.

Makes The Knight's Shade and Siblings non-hostile and deal no contact damage. They can always be destroyed in one hit. Siblings will return back into the ground after a short period. Void Tendrils will no longer appear, and the player can safely swim in any pools that previously had Void Tendrils in them.

This charm replaces Kingsoul and cannot be unequipped. However, it no longer has Kingsoul's effects, nor does it use any notches.

Void Heart also allows entry to the Pantheon of Hallownest.

After it is obtained, this charm unlocks new dialogue with Confessor Jiji, Steel Soul Jinn, The White Lady, Hornet, and Bardoon.

How to Acquire
Found in the Birthplace, an area that is located at the bottom of the Abyss on the left side. Its entrance will only open if the Kingsoul charm is equipped. At the end of the area is an egg which, upon being hit with the Awoken Dream Nail, reveals a memory of the Knight involving vessels and the Pale King.

Trivia

 * Void Heart is the only charm that does not use up a charm notch and is also the only charm that cannot be unequipped.
 * Void heart is automatically equipped once obtained without the need to sit on a bench.
 * When Void Heart is obtained, a distorted version of the normal jingle for getting items is played.

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