Merrow

A merrow on the party's side is a stretch with one clean version: a coastal druid or sea-cult NPC has bound or persuaded the merrow into a single job, and it is on the party's side until the job is done. Merrows are Chaotic Evil and speak Abyssal, so the alliance is transactional. They want something specific, usually meat or a relic, and they will help only as long as the prize is in front of them.

Use the merrow as a transport and an underwater striker. Amphibious plus 40 ft. swim plus a Harpoon that drags Large or smaller creatures 15 feet means the party finally has someone who can yank a fleeing enemy off a boat or pull a drowning ally up from a wreck. In combat, the Multiattack of two attacks (Bite, Claw, Harpoon in any combination) gives the merrow flexibility: open by harpooning the boss off the ship's rail, follow with Bite for the Poisoned rider, then drag the next target with the second harpoon next round.

Out of water the merrow is useless. 10 ft. walk and 45 HP at AC 13 means once it is on the deck or up the bank, it is a slow, fragile mess. Keep it in the water during fights. If the party tries to take it into a city or temple, expect refusals at every gate and the merrow itself complaining loudly in Aquan that the air is wrong.

When the agreed job is done, the merrow leaves immediately. If the party tries to renegotiate, it goes home and tells everyone that humans cannot be trusted. That has consequences for the next coastal port the party visits.

A merrow is an aquatic ambusher with a hand spear. Its whole identity is the harpoon: a 60-foot ranged attack that, on a hit, drags any Large or smaller target 15 feet straight toward the merrow. Run encounters around the geometry of that pull. Set the fight on a pier, in a flooded cavern, on a riverbank with the merrow underwater. The harpoon turns the battlefield into a shoreline and the party into things to be reeled into the deep.

Open from the water. The merrow has 40 ft. swim, 10 ft. walk, and Amphibious, so it should never start the fight on land. From beneath the surface or behind a rock, throw a Harpoon at the lightest-armored PC at +6, 11 piercing on a hit, then pull them 15 feet toward the water. If the pull ends in the drink, the next round opens with a grappled wizard underwater and a merrow Multiattacking with Bite plus Claw at +6, where the Bite also Poisons until end of merrow's next turn. That is a near-guaranteed downed caster by round two unless the party has water mobility.

Mix the action economy. Multiattack is two of any combination, so a smart merrow opens with Harpoon to drag, then Bite for the Poisoned condition, then Claw for raw damage. Two harpoons in one turn against the same target is also legal and very nasty if the first hits, since the second pulls the same target another 15 feet. Run merrows in pairs and have them harpoon two different PCs into the same kill zone.

Merrows are Chaotic Evil and not particularly brave. Once Bloodied (22 HP), the merrow disengages by swimming away at 40 ft. underwater and harassing from range until the party gives up the chase. They do not negotiate above water and only sometimes do below, in Aquan or Abyssal. Most parties cannot pursue. Plan for that.

Make the players roll Athletics or Acrobatics to keep their footing on a wet pier the round before the harpoons come out. Once they know the floor is slick, the pull lands harder.