Cecaelian Siren

Amphibious. The siren can breathe air and water.

Magic Resistance. The siren has advantage on saving throws against spells and other magical effects.

Spellcasting. The siren is a 12th-level spellcaster. Its spellcasting ability is charisma (spell save DC 16, +8 to hit with spell attacks). The siren has following spells prepared:
Cantrips (at will): dancing lights, ray of frost, thaumaturgy
1st level (4 slots): faerie fire, grease, magic missile, ice knife
2nd level (3 slots): crown of madness, hold person, misty step
3rd level (3 slots): lightning bolt, sleet storm, wall of water
4th level (3 slots): evard's black tentacles, summon elemental
5th level (2 slots): antilife shell, enervation, hold monster
6th level: chain lightning

Innate Spellcasting. The siren can innately cast the following spells without any material components. Its spellcasting ability is charisma (spell save DC 16, +8 to hit with spell attacks).

At-will: Arms of Hadar, Charm Person
1/day: Hallucinatory Terrain

Tentacles. Melee Weapon Attack: +7 to hit, reach 15 ft., 1 target. Hit: 10 (2d6 + 3) bludgeoning damage. On hit, the target is grappled (escape DC 15). Until this grapple ends, the target is restrained and the siren cannot use its tentacles on another target.

Harpoon. Ranged Weapon Attack: +7 to hit, range 20/60 ft., 1 target. Hit: 6 (1d6 + 3) piercing damage, 7 (1d8 + 3) piercing damage if wielding in two hands.

Otherworldly Call. As an action, the siren begins to sing. Each creature within 300 ft. that can hear must make a DC 20 Wisdom saving throw. On a failed save, the creature is charmed. While charmed this way, the creature perceives the siren as the epitome of what it finds beautiful. It feels an irresistible draw to the creature. On its turn, it must use its action to move as close as it can to the singing with no regard for its own safety - or the safety of others around it. If the siren attacks the creature, or the creature takes damage, it can repeat this saving throw. On a successful save, a creature is immune to this song's effect for 24 hours. The siren can end the song with a bonus action.

Deep's Kiss. The siren chooses a creature it has charmed within 5 ft. and moves in to kiss it. The creature must roll a DC 16 Constitution saving throw. It can choose to willingly fail this saving throw if it accepts the kiss. On a failed save, the creature's lungs are filled with brackish water and it is considered drowning (see "Suffocating" on page 183 of the Player's Handbook). Another creature within 5 feet may use its action and force the target to cough up the water with a success on a DC 10 Wisdom (Medicine) check.