The hackers, who illegally broke into HBO's computer networks, claim to have taken 1.5 terabytes of data, including the fourth episode of season seven, which aired in the US on Sunday (06Aug17), and was released illegally on Friday (04Aug17), days after a script from the newest instalment of the fantasy series was published on the Internet.
The broadcaster has said it continues to investigate the incident.
The attackers directed their demands at the head of HBO, Richard Plepler, giving him a deadline of three days to pay the ransom or risk further leaks.
The hackers demanded "our six-month salary in bitcoin", claiming they earn $12m to $15m a year from blackmailing organisations whose networks they have breached.
However, the hackers have not made public how much they want. The dump contained technical data detailing HBO's internal network and administrator passwords, draft scripts from five Game of Thrones episodes, including this week's instalment, and a month's worth of emails from HBO's vice president for film programming, Leslie Cohen.
An individual using the name "Mr. Smith" posted a fresh cache of stolen HBO files, including some apparently related to the show "Game of Thrones", online Monday, part of what the purported hacker has claimed is a much larger trove of stolen HBO material.
Company documents and video episodes of other HBO shows were also shared.
One document appears to contain the confidential cast list for Game of Thrones, listing personal telephone numbers and email addresses for actors such as Peter Dinklage, Lena Headey and Emilia Clark.
The tally tops the 10.1 million that turned out for the season premiere last month, a number that in and of itself was a quantum leap for the 8 million-plus audiences that was the HBO series' previous bests.
In the video letter Monday, the hackers claimed that HBO was the 17th company to be targeted by them and that only three previous targets refused to pay, according to the AP.
"We take this breach very seriously and have immediately initiated forensic investigations at our and the technology partner's end to swiftly determine the cause".