Hedge fund giant Millennium Management has reportedly lost one of its top-performing portfolio managers from its flagship quant unit Azur, with his proposed departure sparking a legal battle over restrictive covenants, according to eFinancial careers.

Romain Castillon, a key profit driver within the New York-based Azur team, is believed to be heading to rival GSA Capital, the London-based quant fund known for its lucrative partner payouts and global reach. The report cites unnamed sources as saying that Paul Mayer, Azur’s Chief Technology oOfficer, may also be making the move, though neither firm has confirmed the exits.

The high-profile departures come amid reports that Millennium has initiated employment tribunal proceedings in Geneva, a jurisdiction often used in hedge fund disputes over non-compete and non-solicit clauses. Details of the case remain private, but similar legal spats in the industry have delayed or restricted PM transitions to competitors.

Castillon joined Millennium in 2019 from Squarepoint, and like Azur founder Jonathan Bensimon, is an alumnus of Société Générale’s equities desk.

Azur, founded six years ago by Bensimon and operating across New York, Paris, and Asia, is considered one of Millennium’s largest and most successful quant pods.

Millennium declined to comment. Castillon did not respond to inquiries, and GSA Capital has not confirmed any new hires.