I know I’m a week late replying to this, but this is an event from back when these three monsters originally released back in June 2018. You can see from the end of the event it also teased a new limited-time event coming at the time… Okkult’s Invitation. We used to get these story events every few months so they always had a short story.
For some reason this event was not re-added to the game along with most of the others and they’ve waited until now. You can see on page 5 of the story events the only two others not yet re-added have been put as “coming soon” so we should have literally every one in the game soon.