This achievement unlocks when you obtain the Void Heart charm in the Abyss.
You must first collect the two pieces of the Kingsoul charm. The first is given by the White Lady in Queen's Gardens and requires having the Shade Cloak and defeating the Traitor Lord:
The second part requires getting to the end of the White Palace found in Ancient Basin:
Use the Awoken Dream Nail (acquired by talking with the Seer after having collected 1800 essence) on the statue outside the palace to be taken to a new area. Defeat the Kingsmould in the palace grounds to gain access. Once inside, make your way through several difficult platforming sections. I suggest using the Deep Focus charm along with Grubsong and Grubberfly's Elegy, which gives you maximum healing from focus and gains soul whenever you take damage. Fragile Heart and Longnail are good options if you have two notch spaces left.
This platforming is pretty tough but with the charm setup I mentioned you should generate enough soul to keep yourself healed for as you work your way through. Refer to the following video to see how to get through:
Once you reach the top, attack the Pale King and the second piece of the Kingsoul will appear. Collect it to make the Kingsoul whole. Equip the Kingsoul charm and head to the bottom of the abyss where an opening will appear in the ground:
Drop into this hole and make your way through a maze-like area where you will be attacked by Sibling enemies. There are several dead ends and a single correct path that leads to an open area with a broken spherical item that casts a reflection when approached:
Use your Dream Nail on this object and you will be returned to the Abyss floor. Make your way to the top and when you reach the final ledge, a scene will unfold after which your Kingsoul will change into the Void Heart:
At this point, the achievement will unlock.