Rajah Rabbit's Sash of Vengeance is a Hardmode tier Expert Mode-exclusive accessory that is obtained from the Treasure Bag dropped by Rajah Rabbit. It grants increased jump height, jump speed, autojump and immunity to fall damage as well as 40% increase to movement speed. Also, every 10% of health lost (About 50-60 HP) grants 8% increased damage to most boosted damage type. (If you are melee, your melee damage is boosted etc.)
The damage boost you can get at the lowest health threshold possible is: (At 50-60 HP) +72% increase to your most boosted damage type.
If you are doing a no wing challenge, this accessory can be really useful to acquire as it grants boosts to movement for a majority, also making your life easy while moving around and dodging enemy attacks.