Poland’s KSW is looking to make some history this spring.

The promotion is set to host a champion vs. champion event that is poised to be the biggest event in European MMA history. “KSW Colosseum” will take place May 27 at National Stadium in Warsaw, Poland, which can hold 58,000 fans, but may be configured for as many as 65,000 for the card.

KSW middleweight champion Mamed Khalidov (32-4-2) will fight KSW welterweight champion Borys Mańkowski (19-5-1) in the main event. The fight will take place at a catchweight, though the precise number between 170 and 185 pounds has not yet been finalized.

A packed house for “KSW Colosseum” would put the event up there with the UFC’s two biggest cards. UFC 193 at Etihad Stadium in Melbourne, Australia, had an attendance of 56,214 in November 2015 to watch Holly Holm upset Ronda Rousey in the main event. And UFC 129 at Rogers Centre in Toronto had an attendance of 55,724.

The card will be KSW’s first stadium event. The card will be available worldwide as an online pay-per-view through KSW’s website.

KSW long has been one of the top promotion’s in Europe. Its events have featured recognizable names like five-time World’s Strongest Man winner Mariusz Pudzianowski, UFC standout Karolina Kowalkiewicz, Melvin Manhoef, Alexander Gustafsson, Francis Carmont, Matt Lindland and Houston Alexander among many others.

The 36-year-old Khalidov is one of Poland’s MMA pioneers and arguably the promotion’s biggest star. He has 13 straight wins. He held KSW’s light heavyweight title from 2009-2011. After moving to middleweight, he won his second KSW title in 2015. His resume features wins over Lindland, Jesse Taylor, Rodney Wallace, Kendall Grove, Manhoef and longtime KSW champ Michal Materla.

The 27-year-old Mankowski won the KSW welterweight title in 2014. His fight against Khalidov could prove to be somewhat of a family affair, given his championship win came by submitting Khalidov’s cousin, Aslambek Saidov. He has defended his title three times against David Zawada, Taylor and, most recently, John Maguire.

“KSW Colosseum” also features Pudzianowski (10-5) against an opponent still to be named; the long-awaited debut of Olympic bronze medalist Damian Janikowski (0-0), who still awaits an opponent; and a heavyweight title fight between Fernando Rodrigues Jr. (11-2) and Marcin Rozalski (6-4).