#1 The Fiend being impervious to pain
Part of what made The Fiend character so unique was the fact that he's seemingly impervious to pain.
Sure, other Superstars would get some offense in against him every once in awhile, but it would always result in The Fiend getting right back up for more. He took punishment almost like nothing had even happened to him. It was almost like the character enjoyed the pain and wanted more of it.
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WWE even started teasing that The Fiend had a secret weakness, but they became mute on the subject immediately after introducing it. They also seemed to stop Rabblin Rabbit issuing warnings or trying to escape the Funhouse as well.
Unfortunately for The Fiend character, WWE became very inconsistent with this particular plot point and just used whatever was convenient for the storyline. That's probably why The Fiend popped up after his surprising loss to Goldberg, but laid down after a surprise spear from Roman Reigns at SummerSlam. It has literally just been about what the moment called for and Wyatt was their means to an end.