Anesthetic. In the right conditions (such as murky water), the victim must have a passive perception of 15 or higher to notice the leech (even after it attacks).

Ambush.  The leech gets Advantage on attacks against an unsuspecting target.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target (creatures without blood are ignored). Hit: 7 (2d6) necrotic damage and the leech attaches to the target.  While attached, the leech doesn't attack. Instead, at the start of each of its turns, the target loses 7 (2d6) hit points due to blood loss.  The leech gains temporary hit points equal to that amount. 

With each drain (after the first attack), the victim must succeed on a Constitution save (DC 15) or become incapacitated.  If the victim is incapacitated, the leech will not intentionally kill it but will return for periodic feeding.

The leech can detach itself by spending 5 feet of its movement. It does so after it drains 20 hit points of blood from the target, or the target dies. A creature, including the target, can use its action to attempt to detach the leech by succeeding on a DC 13 Dexterity ability check, or by applying fire.