Call it coincidence, call it fairy-tale magic, call it corporate synergy, call it -- all right, it's definitely corporate synergy. But call it what you will, it's bringing Marvel's Avengers Vs. X-Men to the season finale of the ABC fantasy drama Once Upon a Time.

For those not in the know, the hit TV series centers on bails-bonds collector Emma Swan, who’s led to a town in Maine where fairy-tale characters have been sentenced by the Evil Queen's curse to lead an ordinary existence with no memories of their extraordinary natures.

Sunday's season finale isn't the first time Avengers Vs. X-Men has made a cameo on Once Upon a Time: The first issue was earlier glimpsed in the bed of the sleeping Henry (played by Jared S. Gilmore), the boy whom Emma gave up for adoption 10 years earlier and who's the first to discover the town's secret. That same issue of AvX pops up again Sunday in "A Land Without Magic" along with Henry's storybook and other belongings. Perhaps it's not Emma Swan but rather Captain America or Cyclops who holds the key to breaking the curse!

Okay, it's not, but that kind of transmedia crossover would pretty much trump any of those comic-book prequels everyone's churning out. The season finale of Once Upon a Time airs Sunday at 8 p.m. ET/PT on ABC.