In Hollow Knight there is a door in the Abyss that is locked until you have acquired the King’s Brand. The King’s Brand essentially works as a key to unlock the door and that is the only real use for this item. There is a but more lore behind the item but I will leave that to the lore experts.
Requirements:
How to find King’s Brand?

- Start at the King’s Station in the City of Tears and make your way into Kingdom’s Edge by going under the broken wall.
- If you have not been to the King’s station before click here: https://youtu.be/AxxyeLrbG4E

- From Kingdom’s edge drop to the bottom and head to the right

- Jump up the first opening going up

- Keep following the path straight up until you reach an area where the Monarch Wings are required

- Sit at the bench to the left which is behind a breakable ceiling

- Get ready for a fight with Hornet. She is tougher than the first time you fought her and is guarding the King’s Brand

- Follow the path until it ends where you will find the King’s Rand

What Next?
You can now take the King’s Brand down into the Abyss. While there you can upgrade the Howling Wraiths spell to Abyss Shriek, get the Shade Cloak ability and later upgrade the King’s Soul Charm to the Void Heart charm for the “Good” ending