Actions

Bite: Melee Weapon Attach: +2 to hit. Reach 5 ft., 1 creature. Hit: 1d4 piercing damage and the leech attaches to the target. While attached, the leech doesn't attack. Instead, at the start of each of the leech's turns, the target looses 1d4 hit points due to blood loss.
The leech can detach itself by spending 5 feet of its movement. It does so after it drains 8 hit points of blood from the target or the target dies. A creature, including the target, can use its action to detach the leech.

Leeches can be found swimming through murky waters and crawling through the mud. In some places people use them for medicinal purposes. Their gigantic variation is often encountered in swamps and sometimes on other planes of existence where the planar magic changes their physical size.