Hollow Knight’s true ending — the Embrace the Void ending — requires collecting the Void Heart charm from a sequence that most guides describe mechanically: defeat failed champions, reach the Abyss, obtain the charm. What the guides typically understate: the Void Heart is not a power-up. It is the Hollow Knight accepting what they are.
The Hollow Knight was created to be hollow — empty of will, incapable of thought, so that the Infection could not possess a mind that didn’t have one. The plan failed; the Hollow Knight had enough will to feel the Infection and not enough to contain it. The Void Heart is the protagonist acknowledging this kinship and accepting it.
The true ending’s final boss phase — the Radiance fought inside the Hollow Knight’s dream — is only possible because the protagonist entered the Hollow Knight’s mind through the Dream Nail. The protagonist fights the Hollow Knight’s contained nightmare from inside.
Team Cherry built the true ending to require the protagonist to occupy the villain’s interiority — to fight the contained evil from the perspective of the person containing it. The Void Heart is the key that makes this position available.
200 hours in and I never caught this. The developers really reward obsessive players.
On my first run and now I have to go back and look for this. Worth the detour.