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I've put some thought into the pirate city and units. Saruman will have a better grasp of the lore and other nuances.
So, the city is called the Pirate Lair. In many games featuring pirates, their cities are called "lairs." So, I didn't just pull this out of thin air.
The warrior hero will be the Captain. I think that's obvious. I haven't thought of what to call the others yet.
Creatures:
1. Cabin Boy - Sailor - Sea Dog
Ordinary pirates who specialize more in sailing the ship than in combat. Nevertheless, they can fight with sabers. Having these warriors in the hero's army increases water movement.
Looks like a man in a tattered telnyashka, armed with a saber.
2. Brigand - Pirate - Buccaneer
The boarding party of the ships. Unlike the sailors, they fight much better and receive bonuses to attack and defense during sea battles. Buccaneers can throw daggers, but they only have three charges.
Looks ala Jack Sparrow.
3. Thug - Bruiser - Bonecrusher
The captain's personal guard, recruited from the toughest and most crazed guys. They compensate for their weak armor with high damage and great speed. Bonecrushers have two attacks and possess an unlimited number of retaliation strikes.
Looks like an overly muscled guy with a bare torso, a bandana on his head, and a massive sword.
4a. Scum - Rot - Crawling Scum
It is believed that the souls of drowned pirates dwell within these creatures. With modest damage for their level, the scum compensate with numerous immunities. They have no morale, are immune to death magic, and take half damage from fire and cold. Crawling scum have the ability to regenerate their damage.
Looks like an amorphous mass of rotting seaweed, vaguely resembling a human figure.
4b. Pirate Mage - Renegade - Fallen Mage
Pirates often captured various magical items and spellbooks. Over time, some among the sea robbers managed to master magic. Additionally, Wizards who fell into banditry often join the pirates. On the battlefield, pirate mages fire lightning from wands, while fallen mages can cast fireballs.
Look like the pirate mages from MM8.
5. Siren - Fury - Siren Queen
The legendary sirens were forced to form an alliance with the pirates as they were on the brink of extinction. Pirate captains love using the sirens' abilities to lure in enemy ships. A unit attacked by a siren may switch to the enemy's side for one turn, as if subjected to a hypnosis spell.
Looks like a girl with bird wings.
6a. Kraken - Trapper - Blood Kraken
Land-based giant squids are very valuable allies. Life on land forced them to grow a shell, making krakens as armored as knights. Using their many tentacles, krakens attack all enemies in adjacent cells and do not receive a retaliation strike.
6b. Cannoneers - Bombard - Mortar
Pirates sometimes attacked dwarf ships and noticed that they were protected not by pathetic ballistae, but by cannons. These cannons were immediately adopted for use on pirate ships. Cannoneers are two pirates carrying a portable cannon. They don't have much health for their level, but the cannon shot deals substantial area-of-effect damage.
As described above — two pirates with a portable cannon.
7. Cyclone - Chaos Cyclone - Black Storm
When two elements — water and air — converge, it gives birth to Cyclones. This happens on the open sea during the strongest storms. Mages working for the pirates put in a great deal of effort to put these beasts in their service. Cyclones are immune to electricity and cold and take half damage from ranged attacks. Their attack can trigger a lightning strike. Any creature attacking a Cyclone takes electrical damage (similar to Fire Shield).
Looks like a giant storm elemental with demonic features.
My imagination is running dry for the eighth level.