Blood Drinker. Half the damage that the blood fiend does, heals it. Its claws are magical and deal an extra 13 (3d8) necrotic damage on a hit (included in the attacks).

Feast. Melee Weapon Attack: +8 to hit, reach 10ft. One target. Hit: 8(1d8+4) piercing damage and the target’s Constitution score is reduced by 1d4. The target dies if this reduces its Constitution to 0. Otherwise, the reduction lasts until the target finishes a short or long rest. This adds temporary Hit points for the blood fiend equal to 4 times the constitution stolen. The Blood fiend may never have more than 15 temporary hit points.

Blood Drinker Blade

Weapon (dagger), very rare

During Combat, Whenever the blade tastes blood (deals damage), it gains a +1 bonus up to a maximum of +3. When the blade is full, it glows a crimson and black glow and deals an extra 1d6 necrotic damage. Once drawn, the blade can not be sheathed until it drinks blood. All bonuses fade when combat ends.

Hidden to identify

All necrotic damage dealt adds to a health pool with a max of 100hp When this health pool fills up, the blade drips and pours blood, taking 1 turn (6 seconds) to form a Blood Fiend who only seeks more blood. This also removes any bonuses the blade currently has.