Merrow

Merrow haunt coastal waters, preying on fisherfolk, merfolk, and any other edible creature that crosses their path. These savage monsters snatch and devour unwary prey, hauling drowned corpses back to their underwater grottoes to feed.

History
Merrow are monstrosities with demonic origins; it's not yet established whether Exandrian mythology is quite the same as in the 5th-Edition Monster Manual, in which merrow were created when a group of merfolk were pulled into the Abyss by the Demogorgon and warped over generations into hulking, evil monstrosities.

Some of the merrow off the Menagerie Coast and in the watery tunnels beneath Wildemount have learned to harness elemental magic, sometimes in the service of some deity of the Elemental Plane of Water, and some of these shallowpriests become leaders of merrow clans, urging them to raids on coastal communities or to build sea-floor shrines and temples to prepare the way for portals to their god's realm.

Pools of Wittebak
Mysteriously, there are merrow deep in the Pools of Wittebak, where the gnomes had their Engine Core.

Chapter 10
In, the Mighty Nein defeated four merrow guards underneath the Gentleman's safe house in the Labenda Swamp, with the aid of Calianna. In, they encountered more of the creatures deeper in their lair and defeated a handful of them.

The Mighty Nein defeated two merrow and a shallowpriest near the wreckage of the Tide's Breath whilst exploring the Diver's Grave.

Merrow

 * Senses: Darkvision.
 * Languages: Abyssal and Aquan.
 * Amphibious:  The merrow can breathe air and water.
 * Attacks:
 * Multiattack: The merrow can make two attack, one with it claw and one with its bite.
 * Claw: The merrow can make a claw attack.
 * Bite: The merrow can make a bite attack.
 * Trident: The merrow can attack with its trident and can pull a creature 20 feet toward itself.

Merrow Shallowpriest

 * Senses: Darkvision.
 * Languages: Abyssal and Aquan.
 * Amphibious:  The merrow can breathe air and water.
 * Attacks:
 * Spellcasting: The shallowpriest is a 6th level spellcaster (save DC 13) and can cast the following spells:
 * Call Lightning
 * Thunderwave