Royal Hyrulean Fabric
Royal Hyrulean Fabric is a key item from Tears of the Kingdom.
Tears of the Kingdom
"Tough fabric bearing a royal Hyrulean design. It looks like it could be used to make a paraglider."
The Royal Hyrulean Fabric is a piece of paraglider fabric in burgundy and dark blue, with gold detailing including the Royal Crest of the Kingdom of Hyrule. The fabric can be applied to Link's paraglider by Sayge at the Kochi Dye Shop for 20 rupees.
It can be found in a Treasure Chest in the Lockup of Hyrule Castle, within the large cell where the Stalnox and Hylian Shield were in Breath of the Wild. The cell contains a Gloom Spawn, which if destroyed spawns a Phantom Ganon. The Gloom Spawn cannot pass through the cell door, so Link can kill it with Bomb Flower arrows without putting himself at risk from it, however the Phantom Ganon can exit the cell this way and fight Link in the corridor. The cell itself can only be entered to access the chest by breaking the sediment which forms part of the ceiling.