Source: Bigby Presents - Glory of the Giants

Weapon (Any Sword), Very Rare (Requires Attunement)

The hilt of this sword bears a carnelian engraved with the blood rune.

You can add your Constitution modifier (minimum of +1) to the damage rolls of attacks made with this weapon.

Invoking the Rune. When you target a creature with an attack using this weapon, you can invoke the sword’s rune, causing it to flare with crimson light and infusing your attack with bloodthirsty precision. You then spend and roll one of your unspent Hit Dice and add the number rolled to the attack roll. You can choose to invoke the rune after rolling the d20.

If this attack hits, you can also spend and roll any number of your unspent Hit Dice and add the total rolled to the weapon’s damage.

Once the rune has been invoked, it can’t be invoked again until the next dawn.