The Religious Sect of Sea God

A series of port rebellions from a group of historically seafaring clans who have fallen under the control of an ancient sea monster living deep under the continent with powerful magic. She uses her magic to grant warlocks powers and use them as instruments of her will to convert or coerce the portsfolk into uniting under her banner and bringing sacrifices from inland to be taken to a small island in the sea where her children - gigant hermit crabs with tentacles, pincers, fangs and many tails. They seek to pillage the surrounding lands and farms, taking all livestock and people and forcibly dragging them to sea as sacrifices, turning the area around the ports into a new borderland frontier. In response the Imperial house against dictatorship sails a small fleet into the harbour at night disguised as the warlocks of the isle, and butcher the guards, allowing the main infantry force to take the first port with relative ease. During the night however, the sea monster's children attack the port, devastating the legion and dragging them into the ocean to be drowned and consumed, those who escape are ambushed crossing the borderlands by a group of warlocks disguised as reinforcements and slaughtered. They make a deal with Flyyal the anvil for the secession of the borderlands as neutral territory and the exchange of wood for non-aggression to allow them to build a fleet.
Using the peace and the traded wood, the Guardians of the Deepwater build a great floating castle, a giant ship with five roars of oars on either side and a great mast draped with a dark green sail, ontop it embroided the visage of their mistress. It takes weeks to construct the vessel, with many raids west along the coast into imperial territory for resources and sacrificial people and livestock. The ship is loaded with the warlocks and a trained militia, departing along the coast heading west toward the great port of the Imperial House, intent on razing it in the name of the Deepwater's.
They sail the floating fortress parallel to the cove in which the port city is built, and bombard the harbour wall, then sail smaller vessels in to break the sea wall apart to allow entrance closer, skirmishes are fort as the Deepwater cultists attempt to break the stones and drag them away and imperial sorties try to stop them and rebuild, whilst constructing siege weapons of their own. On the fourth day, two monsters manage to undermine a section of the wall and drag into the sea, along with a sorting party who are eaten alive in full view of the city, then the fortress manages to bust through the wall and reach the harbour before it is sunk by Imperial artillery, forcing the cultists, warlocks and great monsters to storm the port walls with ladders, they are repulsed twice before the a warlock manages to bust down the gate with magic, and they storm the gatehouse, take the walls and begin fighting street by street as Imperial commanders muster blockages, and strong points, covering the main retreat to the secondary walls. When darkness began to fall, the remaining defenders retreated from the lower levels and into the secondary walls. During the night the walls were subject to a number of small probing assaults, with ladder parties lead by a warlock, some with grappling hooks. Throughout the night, a small defensive line was constructed along the line of houses, before the moat protecting the second wall, making a rudimentary barricade incase of a counter-attack. Unbeknownst to the attackers, the sewers of the lower city contained a number of secret tunnels, with during the night were filled with a reinforcement legion of Household guard. On the second day, a sea monster accidentally broke into the sewers and was killed by a group of legionaries, who used the new passage to poison some of the food supplies of the invaders who had begun shifting food into localised warehouses to ease supply issues. A few of these were even burnt in one off raids. Meanwhile, the invaders brought up their siege engines closer to the walls and began shelling the perimeter and watch towers, this was followed by a late afternoon assault on several key locations across the wall, which were repelled with relative ease.
On the second night of the wall siege, a second sea monster managed to catch a raiding party and draw enough attention for the invaders to discover an entrance into the sewers, then an expedition was sent underground and began fighting with some of the legionaries, who mistakenly thought that their whole legion had been ambushed and surrendered, telling their captors about a secret way beyond the second wall, hoping that it would be heavily guarded. The legionaries were slaughtered and their uniforms taken. The commander of the invasion was a powerful warlock, who cast some spells to change his appearance and voice into the likeness of the captain, leading his force inside the sewers, through the secret passage and through the gate, after charming the guards with magic. Once inside, he managed to find his way to the third tier and strapped up a series of retreat banners, causing confusion within the defenders, leading to some retreating from the wall and others staying in position. Recognising the confusion, the leading of the engineers awoke the troops and lead a bold charge on the main gatehouse, personally climbing a robe lodged with a grappling hook, opening the gate and leading the charge through the second tier. After the slaughter was over, the master builder was promoted by the commander, who managed to sneak back down, losing half of his team.