It was always going to take a special challenge to bring Amanda Nunes out of retirement.
The Brazilian left the UFC in 2023 as the most successful women's mixed martial artist of all time, and with a strong claim to be the greatest ever, regardless of gender.
A former featherweight and bantamweight champion, Nunes is the only woman to hold UFC titles in two weight classes simultaneously, and the only fighter, male or female, to successfully defend both belts.
During Nunes' remarkable run, she beat some of the greatest names in MMA history, such as Ronda Rousey, Cris Cyborg and Valentina Shevchenko.
After Nunes was eventually dethroned by Julianna Pena in one of the biggest upsets of all time, she then regained her title in emphatic fashion.
Nunes was so dominant, she ran out of challengers and left the sport feeling like she had completed MMA.
"When Kayla signed with the UFC and I saw the division with some new girls it excited me," said Nunes.
"Because I cleaned both divisions and didn't have anything to do any more. That was when I was sure I wanted to do this again."
American Harrison is a two-time Olympic gold medallist in judo who became a two-time PFL lightweight world champion after transitioning to MMA.
