Create Bloodstone

You perform a ritual to imbue the gem used for the spell's casting with blood magic. To do so, you spend one hit die as part of the casting the spell. For the duration, you can not regain hit die expended in this way. Your current hit points and hit point maximum are reduced by 2 for each hit die expended.

The bloodstone holds the hit die you expend into it, and uses them for it's effects. A bloodstone is a magical object that can be used in one of two ways:

Heal: As a bonus action, a creature holding the bloodstone can restore hit points to themselves or a creature they touch. The amount of hit points restored equals one or more of the hit die imbued into the stone. once used in this way, the die are expended.

Harm: As a bonus action, a creature holding the bloodstone can use it to imbue their next weapon attack with additional damage. Until the creature's next turn ends, the next time they hit with a weapon attack, the attack deals  additional necrotic damage equal to one of the hit die imbued into the stone. A hit die used in this way is expended. If no attack hits for the duration, no hit die is expended. 

Once the stone is depleted of hit dice, the spell ends. when the spell ends, the stone becomes non-magical, the caster's is able to regain their hit die, and the caster's maximum hit points reduction immediately ends (although their current hit point are not affected).