Wondrous Item, rare
The Amulet of the Unfallen is a bloodstone set in silver on a thin, but durable chain.
As a bonus action, the wearer or an ally can look at the amulet to regain an amount of health equal to half their hit dice + CON modifier (added per dice rolled). Each ally can do this once per long rest.
If the wearer drops to 0 hit points without having used the amulet, it will automatically trigger and heal for the amount rolled. This only happens once per long rest.