REvil are a group that provide ransomware, and look for hacking groups that will use their ransomware to hold to ransom businesses that they hope to extract large amounts of money from.
REvil has been behind some of 2020 and 2021's noteworthly attacks that made news headlines, including the Kaseya VSA Attack of July 2021, the JBS Foods Attack of May 2021, and it's believed there are links with Dark Side who the Federal Bureau of Investigations (FBI) confirmed were responsible for the Colonial Pipeline networks attack of May 2021.