Bloodbloater

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

Amphibious. The bloodbloater can breathe air and water.

Blood Drain. Melee Weapon Attack: +3 to hit, reach 5 ft., one creature. Hit: 1 piercing damage, and the bloodbloater attaches to the target. While attached, the bloodbloater doesn't attack. Instead, at the start of each of the bloodbloater's turns, the target loses 1 hit points due to blood loss.

The bloodbloater can detach itself by spending 5 feet of its movement. It does so after it drains 4 hit points of blood from the target or the target dies. A creature, including the target, can use its action to detach the bloodbloater.