Sealed Siblings

The second ending. Fight the Hollow Knight with the Void Heart charm (Void) equipped. Partway through, Hornet arrives and binds it - keep attacking until it falls and you take its place with Hornet sealing you in. If you'd rather have the true ending, don't finish it here - Dream Nail it instead for Dream No More. You can reload before the Black Egg to grab both this and The Hollow Knight on separate attempts.