Underwater Medusa

Petrifying Gaze. When a creature that can see the Medusa's eyes starts its turn within 30 feet of the Medusa, the Medusa can force it to make a DC 13 Constitution saving throw if the medusa isn't incapacitated and can see the creature. If the saving throw fails by 5 or more, the creature is instantly petrified. Otherwise, a creature that fails the save begins to turn to stone and is restrained. The restrained creature must repeat the saving throw at the end of its next turn, becoming petrified on a failure or ending the effect on a success. The petrification lasts until the creature is freed by the greater restoration spell or other magic.

Unless surprised, a creature can avert its eyes to avoid the saving throw at the start of its turn. If the creature does so, it can't see the medusa until the start of its next turn, when it can avert its eyes again. If the creature looks at the medusa in the meantime, it must immediately make the save.

If the medusa sees itself reflected on a polished surface within 30 feet of it and in an area of bright light, the medusa is, due to its curse, affected by its own gaze.

Grappler. The Medusa has advantage on attack rolls against any creature grappled by it.

Multiattack. The underwater medusa makes two melee attacks with its tentacles and one with Its drain life.

Tentacles. Melee Weapon Attack: +5 to hit, reach 10 ft., one creature. Hit: 7 (1d8 + 3) bludgeoning damage and is grappled

Drain Life. Melee spell Attack: +5 to hit, reach 5 ft., one willing creature, or a creature that is grappled by the Medusa, incapacitated, or restrained. Hit: 13 (2d12) necrotic damage. The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the Medusa regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.