not a specific structure, but an event idea:
Legendary Beast Atlantean - Sea-based boss fight
Based on the Kraken boss from AA you could fight in the first and second seas, this would be a remade version of it to instead be a more randomized boss fight spawn in the middle of the dark sea.
Idea on how this might work
Rarely in the far reaches, the rubble of a ship shattered into pieces could spawn with an atlantean in the middle of the rubble, standing on a whirlpool. This boss would be around 800 - 1000 level and would be the most mangled, monstrous looking of any of the atlanteans.
The surrounding “arena” would be composed of broken apart ships drifting on the waves, which you can jump between to dodge attacks. These broken pieces of wood could have bigger areas on them which people could land on so they don’t always fall into the whirlpool.
The legendary beast atlantean in the middle would follow more preset attack patterns rather than having fully random, customized attacks similar to players like the other ones, but instead would have three different types:
Magical Beast Type - The legendary atlantean in the middle of the shipwrecked whirlpool utilizes magic from the few remains of a mind it has, and rarely even be capable of utilizing lost magics. Legendary Beast Atlanteans with this type would utilize more ranged based attacks which force the player to move around the broken arena, having large endlag as its weak points.
Demented Fighter Type - The legendary atlantean in the middle of the whirlpool utilizes fighting styles, the very few remains of its fragmented, mangled brain it still remembers, which could also rarely utilize lost fighting styles. This version of the legendary atlantean would be more close ranged, frantically trying to rip the player into shreds with the only way to fight that it remembers, leaving weak points in its patterns through attack endlag and outraged barrages.
Tyrannical Combatant Type - The legendary atlantean utilizes weapons from what its barely recognizable mind remembers, being possibly able to wield lost weapons as well. This version of atlantean would be more mixed, having both far and close ranged attacks utilizing their weapons and possibly carrying around cannons to fire barrages at you with.
(Note): There probably wouldn’t be any vitality version of this, since I think that would be more fitting with ghosts instead, which could have some other version of it.
Rewards
When the legendary beast atlantean is defeated, the player would be rewarded incredibly handsomely with a piece of rubble spawning in the middle where the whirlpool used to be, having many dark sealed chests and gold chests within it. Alongside that, legendary beast atlanteans that utilize lost versions of stuff would have the chance of dropping the lost magic/fighting style/weapon that they were using as well.