I know I’m a few days late to the party on this, but I have an idea for a horrifying direction you could take Calvus. Instead of having him mutate into a sunfish, have the infection interact with the power he inherited from Nero and thus leave his abilities, intellect, and raw magical power intact… in addition to granting him new abilities and control over Atlanteans.
How is this horrifying? Allow me to elaborate.
Take King Calvus in gameplay, during the very first time you fought (and almost certainly died to) him.
Now imagine if he fought not like a boss npc, but another player experienced in PvP who has been given his loadout of abilities and spells, and would thus be clever enough to use the throne as cover from your attacks, actively destroy the pillars around the room so you can’t do the same, and not let his attacks get ‘caught’ on the terrain.
Now imagine if, while fighting this smarter Calvus, you also had to deal with swarms of Atlanteans who are under his control, on top of an Insanity 3 debuff from merely being in the same room as him, with all the resulting visual effects and interface freakouts distracting you and messing with your hud in a battle where dodging and keeping track of your opponent is crucial.
Now imagine if you have only one chance to beat him. As in, if he kills you, you’re dead for good and never respawn.
Now imagine that death is the absolute best potential fate in store for you if this nigh-unstoppable boss beats you… and given that you have absolutely no experience with fighting Calvus before he got all these new abilities, you most certainly will lose.
Here’s a link to a character bio I whipped up breaking down what this might look like in the context of the Abyss Sea AU: King Calvus, Herald of the Deep