It’s an end of an era at HBO as longtime chairman and CEO Richard Plepler is leaving the company after three decades. He informed HBO staff of his decision in an internal memo (read it below).

The move comes as parent AT&T has been accelerating the implementation of its plans for WarnerMedia under CEO John Stankey after the DOJ’s appeal of AT&T’s Time Warner acquisition was rejected earlier this week. They include a consolidation of all entertainment brands — including HBO, TNT, TBS and Cartoon Network, as well as WarnerMedia’s streaming service — in one group.

Plepler had been an obvious internal candidate though Stankey approached former NBC chairman Bob Greenblatt for the job. I hear Greenblatt is in advanced negotiations to join WarnerMedia. That likely played a role in Plepler’s exit as he reports to Stankey, and that is a layer that I hear will no longer exist in the proposed new structure. Also expected to depart is Turner president David Levy, who is in a similar position. (UPDATE: Levy too has announced that he would be leaving.)

There are now three top-level HBO-Turner executives to leave since AT&T’s acquisition of TimeWarner, along with Turner CEO John Martin.

I hear in the pending consolidation each unit would have its own programming operation with other departments, including legal, business affairs, etc. likely to be merged.

“Richard is one of the most successful executives in our industry and I have been fortunate to have his support over the last months,” Stankey said. “His vision, energy and passion helped to elevate HBO’s brand to what it has become today. Richard’s impact to our business and on the passionate viewers of HBO’s enduring programming will continue to be felt for years.”

Plepler’s departure also puts an end to HBO’s reign as its own fiefdom. Because of the premium network’s status as Time Warner’s golden goose and main contributor to its bottom line —  in addition to being an awards magnet with its prestige fare — HBO had enjoyed complete autonomy. Insiders referred to the pending consolidation move by AT&T as “Barbarians at the Gate”, with Plepler as the last line of defense in protecting HBO’s independence.

In one of his final acts at the network he tried to protect his employees from AT&T’s initiative to “maximize efficiencies” that involved offering veteran HBO staffers voluntary buyouts.

Plepler was elevated to HBO CEO in 2012. Before that he was co-president.

Here is Plepler’s note:

My dad always gave the best advice. Whenever there was a difficult decision to make, he counseled that since no one could ever have perfect visibility into the future, the best thing you could do was trust your instincts. It has been a touchstone for me throughout my life, and I have found myself returning to it again recently as I think about what is an inflection point in the life of this wonderful company. Hard as it is to think about leaving the company I love, and the people I love in it, it is the right time for me to do so.

In the past weeks, I’ve thought a lot about the incredible journey of this company in the nearly 28 years that I have been blessed to be here. It’s a journey of great pride and accomplishment because so many of you, and many others before us, have made HBO a cultural and business phenomenon. Thanks to all of you, we are today churning on all cylinders both creatively and as a business. Thanks to all of you, I can move on to the next chapter of my life knowing that the best team in the industry remains here to carry on our continued progress and success. As I have said before, this is the team of teams.

It has been the great joy of my professional life to share this ride with you over these many years. And the great honor of my professional life to be your CEO. I don’t have the words to express my gratitude for the support and talent that made our success together possible. But suffice it to say, my love for this place, and for all of you, is deeply a part of me and will last a lifetime. I look forward over the coming weeks to thanking as many of you as I can for the thousands of contributions big and small that have made “this thing of ours,” to quote Tony Soprano, so special. I have told John, who has been nothing but gracious since we spoke, that I would work closely with him to assure a seamless and organic transition.

We’ve created a great and unique enterprise and I know that you will protect its legacy and do all to enhance its future in the years to come.

Know that I will always be cheering loudly, even when I am outside this building, as HBO continues to thrive.

With respect, admiration, and gratitude,