Ever since Kalista was revealed, I got a couple of ideas regarding lore of League of Legends, or at least a few of it's parts. Please note that this is my just spitballing here and whether you take any of this seriously or not is up to your own personal discretion. Now let's take this hypothesis one step at a time, shall we?
As for Kalista by her name and armor design we can guess that she has a bit of Greek inspiration (or maybe a bit more). Now, the only faction in LoL that has a Greek theme would be the Rakkor. With this info, we can guess that Kalista had Mt. Targon origins. Now, I'm not quite sure why a Rakkor woman would join the Shadow Isles and it's King (before the undead thing happened), but let's leave that for another time.
Since the Shadow Isle's fall to what what it is today, Kalista became a vengeful spirit that gathered others like her to her side, seeking those that ached for vengeance. Meanwhile, all the other souls of the Shadow isles came together and formed what was revealed by the Harrowing to be the Black Mist. This mass of death would come upon Bilgewater (as depicted in the picture below) and leave many of it's ports lifeless. Now, let's assume that the Shadow Isles seek out more than just Human souls. If that were the case, then what would stop them from attacking the denizens of the depths? Let's assume that the "untold horrors" mentioned in Nami's lore would be the Black Mist manifesting itself underwater. Let's also assume that the reason Fizz's people vanished without a trace was because they all died, and the reason why there weren't any corpses left is because all the dead bodies were turned into zombie fish-folk, which would give the Shadow Isles a major nautical advantage over the living. Now if this was true, then what's to prevent us from saying that the black, oozing liquid that swallowed Nautilus is the exact same thing, and the reason why Nautilus survived was because he was saved by Kalista, who heard his plight and gave him a chance to enact his Revenge. This theory can be supported by one aesthetic detail, the sickly green lights generated by Titan's Wrath are quite similar to those generated by many of the Shadow Isles champions ( Thresh, Karthus and now Kalista).
Another thing worth mentioning would be the exchange between the abyssal pearl and the moonstone. Nami as the Tidecaller, was supposed to bring the pearl to a landwalker, but nobody would come. The reason for this was possibly because the landwalker that was supposed to bring the moonstone was a Lunari, because of their connection to the moon, but because Diana is the last of her kin and the Lunari were essentially gone for a very long time, nobody told her about the exchange ritual, and so she did not come to meet Nami. This ties us back to Mt. Targon and brings us full circle.
With all of the theories out of the way, I would like to ask of the readers to leave their thoughts below and give any ideas they have to this idea. That would be all, and thank you for reading my very first blog post.