Blood Leech

Swarm: The Blood Leech can occupy another creature's space and vice versa, and the Blood Leech can move through any opening large enough for a Tiny leech. The swarm can’t regain hit points or gain temporary hit points.

Blood Frenzy: The Blood Leech has advantage on melee attack rolls against any creature that doesn’t have all its hit points.

Corrupting Presence: Creatures that start their turn within 10 feet of the Blood Leech must succeed on a DC 14 Constitution saving throw or take 7 (2d6) necrotic damage as the corrupting energy of the Abyss seeps into them. On a successful save, they are immune to this effect for 24 hours.

Regeneration: The Blood Leech regains 10 hit points at the start of its turn if it has at least 1 hit point. If the Blood Leech takes radiant damage or damage from holy water, this trait doesn't function at the start of the Blood Leech's next turn.

Multiattack: The Blood Leech makes two Leech Bite attacks.

Leech Bite: Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) piercing damage, and the target must make a DC 14 Constitution saving throw, taking 7 (2d6) necrotic damage on a failed save, or half as much on a successful one. The Blood Leech regains hit points equal to the necrotic damage dealt.

Blood Drain (Recharge 5–6): The Blood Leech targets a creature in its space or one adjacent to it. The target must make a DC 14 Constitution saving throw. On a failed save, the target takes 14 (4d6) necrotic damage, and the Blood Leech regains hit points equal to the necrotic damage dealt. On a successful save, the target takes half the necrotic damage, and the Blood Leech does not regain any hit points.

Burrow into Flesh: The Blood Leech can attempt to burrow into a creature it has successfully bitten. The target must succeed on a DC 14 Dexterity saving throw or be infested by the Blood Leech. While infested, the target takes 5 (1d10) necrotic damage at the start of each of its turns, and the Blood Leech regains an equal number of hit points. The infestation ends if the target or an ally spends an action to make a DC 14 Wisdom (Medicine) check, successfully removing the Blood Leech.

Parting Gift: When the Blood Leech is reduced to 0 hit points, it releases a burst of corrupt blood. Each creature within 5 feet of the Blood Leech must make a DC 14 Constitution saving throw, taking 7 (2d6) necrotic damage on a failed save, or half as much on a successful one.