Kingsoul

Kingsoul is a charm in Hollow Knight. It requires 5 notches to equip.

Function
When equipped, the wearer slowly regenerates SOUL.

Upon going to the Abyss with it equipped, a breakable floor will appear, which will reveal a cave once smashed. Explore said cave until you find a cracked black orb. Hit your own reflection in the orb with the awakened Dream Nail. After completing the dream, you will obtain Void Heart, which permanently replaces Kingsoul.

While you have it, Kingsoul is a pretty useful charm for exploration and platforming challenges, being superior to Hiveblood in this regard. Unfortunately, by the time you obtain it, it is unlikely you still would have any tough platforming challenges left to face.

How to Obtain
Kingsoul is by far one of the hardest charms to get.

First you must go to the Queen's gardens and find a white tree. Talk to the being within to get the first half of the charm.

Then, after the Dream Nail can pierce any mind, go to the edge of the map and strike a suit of armor to enter the White Palace. Complete the area to get the second half of the charm.

After getting both halves, you obtain Kingsoul.