Blood Darter

Blood absorption. The Blood Darter regains 1 hit point at the start of each of its turns. It regains another hit point for each creature within 15 feet of it, and each of those creatures loses 1 hit point.

Bite. melee weapon attack: +12 to hit, reach 5 ft., one target. hit:  10 (3d6) piercing damage plus 4 (1d8) poison damage. If the target has blood, it loses 3 (1d6) hit points and 1 maximum hit point, and the Blood Darter regains hit points equal to the amount of hit point reduction. The maximum hit point reduction lasts until the creature and Blood Darter both finish a long rest.

Gore jet (recharge 6). The Blood Darter fires a focused jet of blood in a 25-foot line that is 1 inch wide. Each creature in this line must make a DC18 constitution saving throw, taking 21 (6d6) piercing damage on a failed save, and half damage on a successful save. On a successful save, the line does not extend past that creature. The Blood Darter gains temporary hit points for 10 minutes equal to one-fifth the amount of damage dealt.

Puppet (1 / day). The Blood Darter attempts to enter a creature within 5 feet of it. The creature takes 7 (2d6) piercing damage, and must make a DC14 constitution saving throw or be entered by the Blood Darter. The Blood Darter remains inside the creature until the creature dies, succeeds on a DC22 constitution saving throw that it must make once per minute, or the Blood Darter is removed with a successful DC20 medicine check. If the creature fails its once-per-minute Constitution saving throw, it takes 2 (1d4) poison damage. A failed Medicine check made to remove the Blood Darter deals 10 (4d4) damage to the creature. While the Blood Darter is inside the creature, the Blood Darter controls the creature's actions. The creature cannot cast spells while controlled in this way.