HBO

"Blackwater" is the episode that made "Game of Thrones" a phenomenon.

After Sunday's season 4 "Game of Thrones" premiere, the HBO show couldn't be more popular.

It's one of the highest-rated cable shows , its twists make the Internet go crazy , and viewers sell out fan events in minutes .

To find out what helped the series become such a phenomenon, we have to go back to season 2's climatic "Blackwater" episode.

"Blackwater" not only brought "Thrones" to the masses; it was also one of the most costly episodes ever produced for television.

Here's how the epic $8 million episode came together.

Preparing For Battle

According to series creators Dave Benioff and Dan "D.B." Weiss, preparation for the Battle of Blackwater episode started all the way back in season 1.

"This whole story of Blackwater goes back to the first season ... we were supposed to have that battle with Tyrion and we ended up not able to shoot it," Benioff told EW. "We always promised ourselves, we’re gonna have our major battle."

That major battle would turn out to be season 2's penultimate episode, "Blackwater." However, bringing a key conflict to life would be a serious production and financial challenge. Luckily, the creators were able to go to the source for help.

"Dave and Dan gave me the hardest episode of the season," author of the series and writer of the episode George R.R. Martin told GQ. "I think it was their subtle revenge for creating such a difficult-to-produce show."

Hardest episode is an understatement. In the second book, "A Clash of Kings," the Battle of Blackwater is a war by land and sea that blows up a fleet of ships and takes up six chapters. Putting it together would undoubtedly be a challenge — but not as challenging as getting HBO to pay for it.

"You Guys Need A Million Dollars?"

According to E! Online, an average episode of "Game of Thrones" costs $6 million, making it one of the most expensive series on television.

"Blackwater" is not your typical episode, so the creators went to HBO to ask for more money, they told GQ:

View photos D.B. Weiss, Dave Benioff, Game of Thrones More

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Weiss and Benioff understood the importance of "Blackwater" but needed to persuade HBO to pay for it.

Benioff: "We had one really intense conference call with the HBO brass. It was awkward: They said, 'So, what are you guys talking about, an extra $500,000?' We said, 'Noooo....' 'You guys need a million dollars?' 'Ummmm....'

Weiss: "I think we asked for $2.5 million. We got $2 million-something. That's a lot of money in TV."

In comparison, the average cable episode costs $2 million, so the extra costs for "Blackwater" alone equaled the price of one episode.

Where the Money Went

Typically, the show's large budget includes filming on location in places like Northern Ireland and Croatia.

For the "Battle of Blackwater" episode, the large budget went toward a three-part fight sequence that consisted of a large naval battle, beach fight, and castle storming.

Filming for the scene took place in Belfast, where Benioff said they shot every night for about a month.

According to a season 2 featurette, the budget was spread across building ships, visual effects, and more than 250 extras that needed costumes and fight training.