#1 Red lighting for The Fiend
It's not easy to protect a character like Bray Wyatt's Fiend. Sure, when he's not wrestling you can use all of the camera tricks, extremely dark lighting and spooky sounds you want, but things change when he's in the ring.
When The Fiend made his debut at SummerSlam against Finn Balor, the match was barely three minutes long, and The Fiend was there and gone almost before you knew it. WWE didn't use any special effects or lighting tricks during the match, because it was going to be short. They wanted you to see the mask, see the outfit, and know what the character looked like, but not for too long.
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The Hell in a Cell match was almost 20 minutes long. WWE wanted to put a powerful message behind the match, and having your Universal Champion at a complete disadvantage by fighting the unknown inside a giant cage with dark lighting is loud and clear.
The Fiend is a spectacle, and he is to be feared. The match itself showed just how indestructible Bray Wyatt's alter-ego can be. He dealt a lot of punishment out to Rollins, but The Fiend spent the majority of the match taking everything the champion could throw at him, only to get right back up.
The superhuman behaviour of The Fiend was scary enough. WWE decided to make it look like a horror movie by making the arena dark and filling it with dim, red lighting. It truly felt like Jason Voorhees -- you can't put him down with one blow, and you can't put him down with 100. And then, when it looks like he's finally been destroyed, he's back to his feet and smothering you to stand tall.
Whether you agree with the way the match ended or not, and whether or not you had an easy time seeing what was happening is almost completely unimportant. The fact that it looked different was all WWE needed. They can't do this all the time, but in a match like Hell in a Cell, with The Fiend unable to be stopped, WWE obviously wanted it to look like a horror movie, and they did it.