#1 Changing trope
It's no secret that facing The Fiend changes his opponents in one way or another and while WWE doesn't always follow through with it, the storytelling device has been interesting to watch. In fact, between Daniel Bryan's renewed focus, Finn Balor's new attitude and The Monday Night Messiah's fight for change, this element has worked wonders for the company.
It's almost like a get-out-of-jail-free card for WWE when it comes to doing a heel or babyface turn, which makes it very valuable in certain situations. It also casts the Superstar that faced The Fiend in a completely different light and seems to make the fans want to follow the losing character even more.
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In the end, facing The Fiend changes you in some way and while WWE hasn't fully explained it yet, it is a powerful booking tool. Beyond that, it makes every feud that The Fiend takes part in must-watch television as fans try to figure out how the character will change his latest victim.