Blood Spectre

Amorphous. The Blood shadow can move through a space as narrow as 1 inch wide without squeezing.

Blood Veil. The Blood Shadow has a veil of blood surrounding it making it hard to hit on its main body, attack rolls made against it have a -1 penality furthermore ranged attacks made against it are made at desadvantage.

Blood Drain. Melee Weapon Attack +6 to hit, 5ft range, one target, for 1d6+1 necrotic damage, and the target's Constitution score is reduced by 1d4.
The target dies if this reduces its Strength to 0. Otherwise, the reduction lasts until the target finishes a short or long rest.

If a humanoid has its constitution reduced to 0 it dies, and its blood is fully drained of its body,  fully healing the Blood Specter that killed the dead humanoid