The backstory
The 31st edition of The Show of Shows was the first of four consecutive occasions when Roman Reigns main-evented the PPV. By the time 'Mania rolled around, Reigns had turned from a legit crowd favorite to getting booed on a weekly basis.
The original plans for the main event were for Roman Reigns to slay The Beast and capture the coveted championship. This finish ended up getting changed in the middle of the show, as Seth Rollins was called to the back after his loss to Randy Orton, and informed that he would be cashing in during the main event.
The main event of Brock Lesnar vs Roman Reigns for the WWE World Heavyweight Title ended in shocking fashion, as Seth Rollins came out in the final moments of the match and cashed in his Money In The Bank briefcase. Rollins pinned Reigns after a thunderous Curb Stomp to leave WrestleMania 31 with the belt.
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An emotional walk to the back
After the dust was settled, Brock Lesnar left the arena and so did Reigns. He was visibly distraught on what had just happened. He slowly made his way to the back, where he was greeted by Vince McMahon himself, who was actually on the verge of tears.
The Boss and Roman Reigns hugged each other in a light-hearted backstage moment, after which The Big Dog met his family, who were reportedly upset at the last-minute change that had cost Reigns the title.
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The aftermath
Roman Reigns went on to become a multiple-time World Champion, and main-evented three consecutive WrestleMania events, battling against the likes of Triple H and The Undertaker. Reigns left WWE in late 2018 to take on leukemia, which had been hounding him since the past decade, and made his triumphant return to the ring on the road to WrestleMania 35.