Mermen, or more properly “merfolk” according to the sages, are the inhabitants of oceans and seas in the warmer areas of the world being particularly fond of tropical seas. Mermen are much like rural humans, farming and harvesting vegetation on the ocean floor and hunting in the area around their settlements; they hunt for fish and other sea-going creatures. Merfolk have been known to herd certain fish, keeping them corralled in large, spherical netted seaweed pens in much the same way their human counterparts will herd cattle.

Merfolk tend to congregate in small communities, establishing their dwellings in reefs or underwater cliffs and tunnelling many rooms and passages into these natural structures. Rarely, mermen will construct dwellings of natural materials such as seashells, coral, rocks, and materials scavenged from sunken sailing vessels. The community will consist of roughly equal numbers of mermen, mermaids (as the female of the species is called), and younglings. These communities, no matter the location, will correspond to a human agricultural type community in many respects: there will be workshops, residences, pens for the food fish, and storage areas for the harvested sea vegetation. Merfolk communities are guarded by 3d6 giant gar (q.v.), bred and trained by the mermen for this purpose. Merfolk rarely venture from the ocean but can sometimes be spotted sunning themselves on coastal rocks in isolated areas.

In battle, half the merman troops encountered will be armed with dagger and trident. The remainder of the forces will be evenly split between crossbow and dagger; or net, javelin, and dagger. Mermen crossbows are identical in function to the sahuagin crossbow (see monster listing: sahuagin). If the mermen encountered are seeking to capture a surface ship (see below) 25% of the trident-armed troops will also be equipped with grapples that have 50-ft of line attached.

Though mermen tend to avoid humans under most circumstances, they have been known to attack and sink surface vessels to plunder them. Grappling hook-equipped mermen will surface and hurl their grapples up to 30-ft as the other mermen attack exposed crewman on the ship with crossbows and thrown javelins. The merfolk are quite proficient with grapples and will successfully score a hit on a roll of 1-9 on a 1d10. If a grapple hits it will be grabbed and held by 10 mermen, causing the ship to lose 30-ft per round of top speed for each successful grapple attack. If the crew of the ship manages to cut a grapple line it will take the 10 mermen holding the line a round to regroup and return to the fight. Any merman grappling the ship or laying down cover fire is exposed to ranged attack from the ship. Further, fire-based attacks will cause double damage but only for a single round. If the grappled ship is successfully rendered motionless it will be pulled underwater in 4d4 rounds. Any crewmen abandoning ship at this time will be ignored as the ship and mermen sink beneath the waves. Mermen do not take surface dwellers prisoner, but will not normally harm any crewmen in the water—unless the crew of the ship foolishly try to continue the battle.

Merfolk appear as completely normal humans from the waist up and as man-sized fish or dolphins from the waist down. They disdain clothing, though mermaids will often adorn themselves with plundered jewellery and mermen will use any enchanted weapons and protective magic rings or bracers they may gain in their raids. Mermen speak their own language, 50% speak locathah, and if the merfolk dwell near any other intelligent aquatic creatures at least 10% of the community will be able to speak that tongue as well.