I've got a summary of the game's Fairy Tail crossover side mission (it's in the base game)
>The EZ crew gets hired by a woman named Maruha, who's a veteran of the Shooting Starlight guild. She sends them on a mission to stop criminals who are inexplicably appearing on Blue Garden, even ones who are supposed to be dead (basically, every boss you fight in the game).
>The culprit is a man named Submerge, who uses an EG called Dream Catcher that takes figures from people's nightmares and embeds them in androids to turn them real. His most powerful creation is a fake, evil version of Natsu.
>Shiki feels sorry for the androids who are forced to act as war machines against their wills. This surprises Maruha, who believes all humans hate humans.
>During one mission, Maruha is revealed to be in league with Submerse, using the crew to make his nightmare soldiers stronger. She's actually an imposter android called TB 6008 (the real Maruha was locked up by the villains and rescued offscreen). She wants revenge against humans for discarding her on Planet Hook and destroying it after she escaped.
>After a fight with the fake Natsu, the EZ crew is saved by Lucy and Happy, who have been summoned to the EZ world while on a quest. They think Submerse has something to do with it, and assume beating him will send them back.