Cormier (21-1-1NC) flattened the UFC's long-reigning heavyweight champ with a right uppercut out of a clinch.
With the knockout win, Cormier became the second-ever UFC simultaneous two-division champion. The first was Conor McGregor, who accomplished the feat in late 2016.
Cormier's 5-inch height disadvantage and 8-inch reach disadvantage against Miocic proved to be no problems - and he actually weighed a few pounds more than Miocic at weigh-ins on Friday. After getting his hand raised, Cormier played host to a ringside Brock Lesnar who made a decision to crash the festivities, in an apparent hype-job for a future tilt. He has an impeccable resume, both earlier in his career as a heavyweight and now as the light heavyweight champion, against everyone not named Jon Jones.
The buzz immediately shifted from Cormier becoming the new champion to Cormier and Lesnar now being the the fight to make.
Lesnar referred to Miocic as a piece of crap and then said to Cormier, "I'm coming for you mother [expletive]!"
"I was a heavyweight for a long time, and I left this division", Cormier said.
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If the suspension is five months, Lesnar could be back as soon as UFC 232 on December 29 in Las Vegas, but if the suspension is six months, his return would have to wait until 2019.
Cormier and Miocic stood toe-to-toe and traded hands throughout.
Gustafsson faces Volkan Oezdemir on August 4 in Los Angeles, but the prospect of money fights with superstars Jones and Lesnar would probably be too great to overlook for Cormier.
He knocked out the American in the closing moments of round one, adding the UFC heavyweight championship to the light-heavyweight one he already holds.
The bout will headline a pay-per-view card at 7 p.m. that also includes a heavyweight bout between contenders Derrick Lewis and Francis Ngannou.
The 11-fight event kicks off with two fights streaming live on UFC Fight Pass at 7 p.m. ET. He has successfully defended the heavyweight strap in three consecutive bouts.
That's a big, big difference, and could easily be the difference in the fight.