In Dark Souls 3, Undead Bone Shards grant players a boost to an Estus Flask's recovery amount, permanently increasing the number of hit points (HP) recovered from a regular Estus Flask and/or focus points (FP) recovered from an Ashen Estus Flask. The Dark Souls universe has a plethora of items that provide buffs and benefits to the player, but these are arguably among the most important of them. Given how imperative an Estus Flask is to ensure that players don't have to chug multiple valuable healing items at a time, it only makes sense to grab as many of these rare items as possible to make the trek through this grueling game as manageable as possible.