Void

Obtain the Void Heart charm. Assemble the Kingsoul charm (the two King's Soul halves from the Queen in the Queen's Gardens and the Pale King in the White Palace), then reach the Abyss at the bottom of the Ancient Basin - its sealed door needs the King's Brand from Kingdom's Edge (Proof of Resolve's region). Take the Shade Cloak there, then with Kingsoul equipped use the Awoken Dream Nail at the abyssal birthplace to unite the void, upgrading Kingsoul into Void Heart. Grab The Hollow Knight first - once you hold Void Heart you can't get that ending. This is the gate to Sealed Siblings and Dream No More.