SPOILERS BELOW
I watched the whole crossover story a few days ago, it's aimed at a much younger audience and it's a mobile game so as long as you take it for what it is it's kind of nice all the same. You can tell the developers of this game really love XI, they put so much effort into this. The whole thing is 7-8 chapters.
Prishe from Abyssea (no head ornament), Prishe/Ulmia from Vana, Lillisette, Shantoto, Iroha, Cait Sith and Promathia (Shinryu form) are all in the story. As well as characters from other franchises, they come together after being dragged into this new world and help fight off Abyssea monsters attacking a village in the sky kingdoms.
Basically, what happened in Aby is happening in this sky kingdom. Shinryu is sending out his minions to kill people and that is allowing him to absorb them into himself to become stronger.
Shantoto was not dragged there but somehow opens some kind of communication tunnel and so can create doll replicas of herself in this world for various reasons, she creates a massive duplicate of herself and along with Vana prishe they defend the city from the monsters while the others goto fight Shinryu. They really dialed shantoto upto 11 in this, it's hilarious to hear her say the names of all the ancient magics tho.
They are trying to fight Shinryu (promathia), but just as happened before in Abyssea they can't win against him because he is too powerful and so just like with Aby they need one persons help to win. At a point where they are about to be beaten, Caitsith tells everyone to all hold up their traverser stones. That allows the adventurer (you from ff11) to come into their world to help take down Shinryu. You form up with Lillisette to create the wings of the goddess and that blast weakens Shinryu.
ngl got me in the feels when Iroha shouted master!
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The whole set is on the channel but this shows the best part of it, watch from 21 minutes on.