Found in a secret area at the bottom of the Abyss known as the Birthplace. The area's entrance only opens 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. It is after traversing this memory that the Kingsoul is changed into the Void Heart.
Location of the entrance to the Birthplace
Entrance which opens with Kingsoul equipped
Ohhhmmmm... Tiny thing... It evolves beyond that Wyrm. Such union in a single being. A strength before unseen. Would it too challenge nature? It could perhaps defeat it.
Confessor Jiji after defeating the Shade
Ooohhhh. My masters would be impressed... Rare it is for one to come to terms with their regrets so completely, yet you seem to have managed it. What darkness must one wade through to achieve such a thing?
Steel Soul Jinn
This It... A rare It. Has not seen many, so vulnerable, but triumphant. Makes Jinn feel a thing... surprise?
Jinn misjudged... The It is not inferior. Perhaps... different? Different to Jinn. More complete? Different... like masters?
That pulsing emptiness... Truly, it has been transformed by the revelations it found.
Does it... feel anything? Triumph? Or hate? If it does, I cannot sense it.
The fate of our Kingdom, our Hallownest... that future belongs to you now.
Hornet outside the sealed Black Egg
I'm impressed little ghost. You've burdened yourself with the fate of this world, yet you still stand strong.
To break the Dreamer's seals would alone be considered an impossible task, but to accept that void inside yourself, that casts you as something rather exceptional.
Hornet outside the unsealed Black Egg
The path is opened. One way or another an end awaits inside.
I won't be joining you in this. That space is built to sustain your likes. Its bindings would drain me were I to join.
Don't be surprised. I'll not risk my own life in your attempt, though if the moment presents I'll aid as I'm able.
Ghost of Hallownest, you possess the strength to enact an end of your choosing. Would you supplant our birth-cursed sibling, or would you transcend it?
Hornet Dream Nailed
...Could it achieve that impossible thing? Should it?
Remember the past and unite the Abyss
It is not possible to die during the climb up the Abyss after which Void Heart is obtained. Damage can be taken after being hit by spikes, but the Knight's health never runs out.
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.