Private-equity firm Silver Lake has asked a Delaware judge to declare that Carl Icahn and other investors illegally colluded to oppose its $13 billion acquisition of Endeavor Group Holdings. The lawsuit escalates a high-stakes battle over the buyout of the entertainment and sports agency giant, which owns WME and UFC parent TKO Group Holdings. Silver Lake, which already controls a majority of Endeavor, launched a tender offer in April to take the company private at $27.50 per share. Icahn, who amassed a roughly 8% stake, has argued the price significantly undervalues the company and urged fellow shareholders to reject the deal. In its court filing, Silver Lake alleges that Icahn and entities affiliated with Sculptor Capital Management and Aristeia Capital formed an illegal "group" under securities laws to coordinate opposition without proper disclosure.

The firm seeks a ruling that would prevent the defendants from voting their shares or soliciting proxies. The case highlights growing tensions between private-equity buyers and activist investors in merger arbitrage situations. A hearing on the preliminary injunction is expected soon.