Amphibious. The triton can breathe air and water.
Nimble Escape. The triton can take the Disengage or Hide actions as a bonus action on each of its turns.
Innate Spellcasting. The triton's spellcasting ability is Wisdom (spell save DC 12). It can innately cast the following spells, requiring no material components:
1/day each: fog cloud, gust of wind
Multiattack. The triton makes two urchin-spine shortsword attacks.
Urchin-Spine Shortsword. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit:6 (1d6 + 3) piercing damage plus 10 (3d6) poison damage. If the damage reduces a creature to 0 hit points, that creature is stable but poisoned for 1 hour, even after regaining hit points, and is paralyzed while poisoned in this way.
Poisoned Spine. Ranged Weapon Attack: +5 to hit, range 30/60 ft., one target. Hit:5 (1d4 + 3) piercing damage plus 10 (3d6) poison damage.
Some insults don't wash away with the tides. When surface dwellers threaten the safety of triton communities, impede upon Thassa's holiest depths, or steal the treasures of the deep, triton shorestalkers seek vengeance. Using speed and poison harvested from deadly sea beasts, these triton assassins slip into shallow waters and strike when least expected. Often, surface dwellers don't even realize they've been attacked by shorestalkers, chalking disappearances and deaths up to the innumerable…