This achievement is unlocked after defeating Radiance, the game's true final boss.
There are three different endings to the game. All three require that you first destroy the three Dreamers, then fight the Hollow Knight in the Temple of the Black Egg.
Ending 3 (for this achievement): Collect both pieces of the Kingsoul charm so that you can then obtain the Void Heart charm. Near the end of the Hollow Knight fight in the Temple of the Black Egg, Hornet will swoop in and fracture the Hollow Knight's shell with her needle before restraining it. Quickly hit the Hollow Knight with the Awakened Dream Nail (hold
) and you will be taken to a new area where you will fight Radiance, the game's true final boss. Defeat Radiance to complete the game and this achievement will unlock after the ending scenes and credits.
It is possible to get all three endings on a single save file as after reaching an ending, you can continue your save file from prior to engaging the Hollow Knight. Start by defeating the Hollow Knight without having acquired the Void Heart charm, then continue (from your last save point). Engage the Hollow Knight after acquiring the Void Heart charm and after Hornet intervenes, keep attacking the Hollow Knight until it is defeated. Continue (from your last save point) and engage the Hollow Knight again. When Hornet intervenes, use the Awakened Dream Nail on the Hollow Knight to initiate the battle with Radiance.