Sahuagin are fierce, amphibious fish-men that live underwater but emerge periodically to raid coastal settlements. Although the Monster Manual says they “dwell in the deepest trenches of the ocean,” that’s a bit far for even a creature with a 40-foot swimming speed. Those ocean trenches are as far from the coasts as the highest mountains are, and you don’t often hear about the yeti of the Himalayas spending an afternoon staging a raid on Kolkata, or the Tatzelwürmer of the Alps popping down to Genoa for some late-night ravaging. These are distances of hundreds of miles we’re talking about. So chances are, any sahuagin that player characters encounter are going to be denizens of shallower depths. Maybe they’re the border reivers of the ocean kingdom.
When they come ashore to raid, they do so at night, as implied by their 120 feet of darkvision. They can’t come far inland, since their Limited Amphibiousness gives them only four hours of air breathing before they have to return to the water. Unlike, say, merrows, sahuagin can move about on land as easily as any other humanoid.
In this environment, they’re basic brutes. Their Multiattack gives them one weapon or claw attack and one bite attack. Since their armor class doesn’t include a shield, we can presume that they wield their spears two-handed for the greater damage.
Because their Blood Frenzy feature gives them advantage on attacks against any opponent who’s taken damage already, they’ll press this advantage, continuing to attack any enemy they’ve bloodied rather than switch targets, even for opportunistic reasons.
However, sahuagin are not dumb. In fact, their Intelligence and Wisdom are higher than those of the average humanoid. They’ll attack with discipline, in cohesive units, perhaps even splitting up to flank or encircle a group of enemies. And they’ll realize quickly when they’re outmatched, retreating if all the members of their fighting group are moderately wounded (reduced to 15 hp or fewer) and their enemies aren’t hurt even worse. When they come ashore, they’re looking for an easy fight. A hard one isn’t worth the effort. Retreating sahuagin are sophisticated enough to use the Disengage action.
In the water, they don’t fight significantly differently, but they do have a couple of advantages they don’t have on land.
First, they’ll usually be accompanied by sharks. Thanks to the Shark Telepathy feature, any sharks fighting alongside them will show the same intelligence and discipline that they themselves possess.
Second, being naturally aquatic, they can easily recognize when other creatures aren’t naturally aquatic. They’re smart enough to know that a halfling isn’t a sea creature, and therefore, if it’s breathing underwater and swimming as if born to it, there’s some kind of magic giving it that ability. They won’t know automatically who’s casting water breathing, for instance, but as they fight, they’ll be trying their darnedest to figure it out. They know that magic exists, and once they see a player character cast a spell, they’ll draw the logical (though not necessarily correct) conclusion that that same PC is the one enabling his or her allies to breathe, and they’ll focus their attacks on that PC. If they subsequently see a different PC cast a spell, some of them will break off to attack him or her as well. A reader points out that the spell water breathing doesn‘t require concentration. Sahuagin may or may not be aware of this fact, and may focus less or more of their attention on spellcasters accordingly.
Sahuagin in the water are also more tenacious than sahuagin on land. When fighting underwater, they’ll hang in there until a majority of them are seriously wounded (reduced to 8 hp or fewer). After that, they’ll Disengage and retreat if they’re on their own, but they’ll fight to the death at the command of a sahuagin baron or priestess—usually in order to defend a location or object.
The sahuagin baron doesn’t differ significantly from an ordinary sahuagin. It’s stronger, tougher and a little smarter, its Multiattack gives it one more poke with a melee weapon, and it gets proficiency on all the most important saving throws, so it has less reason to fear spellcasters—not that sahuagin fear them to begin with. When any group of sahuagin led by a baron launches a coordinated attack on a PC spellcaster, the baron will take part in that attack, not just hang back and give orders. Sahuagin raids on land don’t usually include a sahuagin baron—not unless they’re launching an all-out attack of conquest.
The sahuagin priestess is primarily a support caster that carries no weapon, although it still has a claw/bite Multiattack. It will always cast spiritual weapon (bonus action) on the first turn of combat, but lacking any useful cantrip to cast along with it, it will
either move up behind an ally and use the Help action to give it advantage on its own melee attack, then retreat again afterward, or simply Dodge. [As a reader called to my attention in the comments, if the priestess gets close enough to the enemy to Help, she risks an opportunity attack when she falls back. That’s not worth it.]
On subsequent turns, the priestess’s choice of spell depends on what else is going on. If the priestess’s allies are focusing attacks on a spellcaster, the priestess casts guiding bolt at that caster to do some radiant damage and also give advantage to the next attacker. (Ending any water breathing spell is the sahuagin’s No. 1 priority.) If it becomes evident that the players’ water breathing doesn’t come from a sustained spell, however, then the priestess casts hold person on the most dangerous-looking opponent who has the Extra Attack feature. If there are two, and the priestess can’t choose between them, it will spend a 3rd-level slot to boost the hold person spell and nab them both. However, that being said, sahuagin priestesses understand the connection between clerical spellcasting and the brandishing of holy symbols, and they also know that spells like hold person don’t work as well on the devout, so paladins get a free pass from this.
The presence of a priestess also keeps a moderately wounded group of sahuagin in the fight longer: anytime all the sahuagin in the group are reduced to 15 hp or fewer, the priestess casts mass healing word to top them back up, then uses either the Help action or the Dodge action, as in round 1, until all its 3rd-level spell slots are gone.
Finally, in any group of sahuagin with both a baron and a priestess, it’s the priestess, not the baron, that’s the de facto leader (whether or not the baron cares to admit it). If a PC tries to start a parley, the priestess will call a halt to combat and cast tongues to allow the two groups to communicate. It’s unlikely that the sahuagin will ever initiate a parley on their own, however, unless they know they’re outmatched and aren’t guarding anything super-important—and unless they have a priestess with them, because they know most land creatures don’t speak their language.
Sahuagin priestesses call a general retreat when they’re moderately wounded (reduced to 23 hp or fewer), unless they’re guarding an important location or object, in which case they defend until they’re seriously wounded (reduced to 13 hp or fewer). Sahuagin barons will always fight until they’re seriously wounded (reduced to 30 hp or fewer), then call a general retreat.