Based on the search results, there are a few things that have happened to Brock Lesnar:

• In WWE Backlash 2023, he lost in the main event of the show.
• He retired from UFC after a match he had in December 2011, where his opponent targeted the midsection of Lesnar. Lesnar may have realized that he needed to step away from the octagon, as his opponents would always target his abdomen, and he would likely always have fear in the back of his mind that the diverticulitis could come back.
• He returned to WWE in 2016, attacking The New Day, The League of Nations, and Kevin Owens, before performing an F-5 on Roman Reigns. He then re-ignited his feud with Roman Reigns, who won the Elimination Chamber match at Elimination Chamber on February 25 to become the number one contender to Lesnars title at WrestleMania 34. At WrestleMania on April 8, Lesnar pinned Reigns to retain the title in the main event. Rumors arose that Lesnar would leave WWE and rejoin the UFC. On April 9, Lesnar re-signed with WWE.
• He returned to WWE in 2021, sporting a cowboy/farmer look with a beard and ponytail, confronting Universal Champion Roman Reigns at SummerSlam on August 21, 2021, as a face.
• He reportedly got injured during the early stages of his loss to Cody Rhodes at SummerSlam in 2023, but was able to work through it and put on one of the best performances of the night. While Lesnar was initially advertised for other shows this year, the current plan is for him to not return until the build toward Royal Rumble in January.

In summary, Brock Lesnar has had a successful career in both UFC and WWE, but has also faced some injuries and health issues along the way. He has retired from UFC and returned to WWE multiple times, most recently in 2021.