Sash of Kings is a rare-quality equipment item that can be found within the instanced mission map Gorilla Grodd's Lab. Once equipped, it unlocks the Egyptian waist costume style.
Source[]
The Sash of Kings can be found inside Treasure Chests encountered within the Gorilla Grodd's Lab mission map.
Treasure chest[]
Style unlock[]
Upon equipping the Sash of Kings, a character unlocks the Egyptian waist costume style, which can be accessed at any time from the Style Window.