Corvex has spent more than a year pressuring Energen to seek a buyer, saying the company had valuable assets in the prolific drilling terrain known as the Permian Basin but that its shares were undervalued. Earlier this year, it nominated four candidates to Energen's board to ratchet up the pressure. The two sides came to an agreement in March, when Energen agreed to appoint two board members and begin a strategic review of its business.

Any deal for Energen could also affect activist Paul Singer's Elliott Management, which took a stake in the company last year and sided with Corvex in pushing for a sale.

Meister founded his own firm, Corvex Management, in 2010 after years learning the activist trade at Icahn Enterprises, where he was vice chairman of the board and principal executive officer.

Carl Icahn is joining his protege Keith Meister in a deal that may ultimately lead to their purchase of oil and gas producer Energen Corp .

One of those new board members was Vincent Intrieri, a former senior managing director at Icahn Capital Management who retired from that firm last year. He served on the boards of several companies on behalf of Icahn during his time there, including Hertz Global Holdings and Transocean, where he still sits, and Chesapeake Energy, where he sat from 2012 to 2016. He joined Energen's board in March.

But the story is not ending there. On Monday, Corvex said in an amended securities ownership filing with the Securities and Exchange Commission that it had a deal to sell 2 million shares of its stake to Icahn and the option for him to buy 2 million more. In total, Icahn would end up with about 4.1 percent, the filing said.

Following the transaction, Corvex will have 8.46 million Energen shares, or about 8.7 percent, about even with fellow top shareholder Vanguard Group, but smaller than the nearly 10 percent it previously held, according to FactSet.

In March, Energen said its board had begun an in-depth review of its strategic options, getting advice from J. P. Morgan and Tudor, Pickering Holt & Co. Corvex said in the filing on Monday that it believes the company's shares are undervalued and that it may "have an interest in joining with other parties to acquire the issuer as part of the strategic initiatives process or otherwise."'

Last year, Singer's Elliott Management joined Corvex in pushing Energen to sell. It holds about 3 percent of Energen's shares, according to FactSet.

Elliott said in March it supported Energen's decision to review strategic alternatives. "We expect that any such review, to be considered truly comprehensive, will be conducted with shareholder input and will include full engagement with interested acquirers," it said.

A spokesman said Elliott wasn't commenting on Monday's news. On CNBC's "Halftime Report" on Monday, Icahn said he hadn't talked to Singer about the deal.

Icahn also said he believed Energen was worth possibly $100 a share and that it might be an attractive purchase for a major energy company, particularly a pipeline operator. The additional pressure from him and Meister seems to be aimed at pushing the strategic review along more quickly.

"We have a lot of cash around. We could buy this company," he said, with or without help from others. "However, I don't presume to feel I'm the right buyer for it because there are such synergies for another company."

But, Icahn added, Energen's board "agreed to do a process on March 7 and nobody's heard about that process yet."

Shares of Energen are up 6 percent in Monday trading.

When Energen announced its settlement Corvex, Meister released a statement saying, "We are pleased to have worked constructively and to have reached a resolution that will bolster the Board with new highly qualified directors as we pursue our common goal of enhancing shareholder value."

A spokesman said Energen declined to comment.

WATCH: Carl Icahn on Xerox