Undead Stirge

Undead Fortitude. If damage reduces the Undead stirge to 0 hit points, it must make a Constitution saving throw with a DC of the damage taken, unless the damage is radiant or from a critical hit. On a success, the Undead stirge drops to 1 hit point instead.

Diseased Blood Drain. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 5 (1d4 + 3) necrotic damage, and the stirge attaches to the target. While attached, the stirge doesn't attack. Instead, at the start of each of the stirge's turns, the target loses 5 (1d4 + 3) +1 per round attached hit points due to blood loss.  
If the target is a creature other than an undead, it must succeed on a DC 12 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

The stirge can detach itself by spending 5 feet of its movement. It does so after it drains 10 hit points of blood from the target or it or the target dies. A creature, including the target, can use its action to detach the stirge.

Regurgitate: As a Reaction when an Undead stirge is attached to a creature and takes damage that does not kill it, it regurgitates diseased blood into the wound, flushing the creature's bloodstream with paralytic disease.  The creature is stunned until the end of its next turn as the disease works its way though the body.

Festering Sore: Due to the necrotic poisoning of the Undead stirge's bite, the round after the stirge is removed from a creature, blood continues to pool from the open wound, dealing an additional 8 (2d4+3) Blood Drain damage at he beginning of the creature's turn.