Clumsy Moves. While the siren is neither flying nor swimming, attack rolls against it have advantage.

Dark Devotion. The ekhidna has advantage on saving throws against being [condition]charmed[condition] or [condition]frightened[condition].

Limited Amphibiousness. The siren can breathe air and water, but it needs to be submerged at least once every 4 hours to avoid suffocating.

Multiattack. The ekhidna makes two attacks: one with its claws and one with its tail constrict.

Claws. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (2d4 + 3) slashing damage.

Tail Constrict. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 12 (2d8 + 3) bludgeoning damage, and the target is grappled (escape DC 14). Until this grapple ends, the creature is restrained, and the ekhidna can't constrict another target.

Shriek. Each creature within 20 feet of the ekhidna that can hear its shriek and that isn't a siren must succeed on a DC 14 Wisdom saving throw or be stunned until the end of the ekhidna's next turn. If a creature's saving throw is successful, the creature is immune to the ekhidna's shriek for the next 24 hours.