Bloodseeker Mosquito

Blood Draw. The mosquito is more likely to attack creatures that have received piercing or slashing damage.

Weakening Saliva. When the mosquito succeeds on a bite or blood drain attack against a creature, it must succeed on a DC 11 Constitution saving throw or suffer a -1 penalty to attack rolls, saving throws, and skill checks until it is healed. This does not affect Plants, Constructs, Undead, or creatures otherwise immune to nonmagical diseases.

Blood Drain. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: 5 (1d4 + 3) piercing damage, and the mosquito attaches to the target. The creature must succeed on a DC 11 Constitution saving throw or suffer a -1 penalty to attack rolls, saving throws, and skill checks until it is healed. 

On subsequent turns while it is attached, the mosquito doesn't attack. Instead, at the start of each of the mosquito's turns, the target loses 5 (1d4 + 3) hit points due to blood loss. A creature must repeat the DC 11 Constitution saving throw against weakening saliva.

The mosquito 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 mosquito.