Stirge

Tiny beast, unaligned

Armor Class 14 (natural armor)
Hit Points 2 (1d4)
Speed 10 ft., fly 40 ft.

Senses darkvision 60 ft., passive Perception 9
Languages --
Challenge 1/8 (25 XP)

Actions

Blood Drain. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: 5 (1d4 + 3) 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 5 (1d4 + 3) hit points due to blood loss.
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 the target dies. A creature, including the target, can use its action to detach the stirge.