#4 Dean Ambrose
Years before Dean Ambrose became WWE World Heavyweight Champion by cashing in his briefcase on Seth Rollins, he was shedding blood on a regular basis as a backyard wrestler in Combat Zone Wrestling. The promotion is notorious for putting on extremely violent and gory matches, some of which take place in backyards and parking lots.
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Ambrose wrestled in CZW from 2009 to 2011 and was known for his incredibly disturbing death matches (hardcore wrestling bouts where DQs, countouts, and other rules don't apply). YouTube has an extensive collection of Ambrose's hardcore backyard wrestling matches for interested viewers to check out, but prepare yourself for a bunch of disturbing footage before you head over to check it out.
Ambrose later signed with WWE and became a part of The Shield, one of the greatest factions of all time. He went on to become a Grand Slam Champion.