Con Drain Stirge

Blood Drain. Melee Weapon Attack: +8 to hit, reach 5 ft., one creature. Hit: 1 piercing 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 2 Constitution Points due to blood loss. Lost Constitution Points will regenerate normally at a rate of 1 every 24 hours.

After combat, the DM should make a secret Constitution Saving Throw using the reduced constitution of all characters who were successfully targeted.  On a fail, the stirge has inflicted a debilitating disease on the target.  After 24 hours, the ravages of the disease will show up, in addition to preventing all Constitution restoration until it is cured, the character will also have their movement speed reduced by half.   The disease can be cured through magical means or it will cure itself after 72 hours (3 days) - though the players do not need to know this.

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