"The Iron Bank will have its due." – Braavosi saying
Game of Thrones viewers have heard of the Iron Bank; it's often mentioned in passing by the characters populating the Seven Kingdoms. Take the Season 7 episode entitled "The Spoils of War": in it, Cersei Lannister and the enigmatic Tycho Nestoris talked at great length about the Iron Bank of Braavos. If you're the kind of fan who has stuck with the television show but never cracked one of the books' spines, then you might have been a little lost. But that's to be expected. After all, who's watching Game of Thrones for all the financial drama?
Well, you might want to brush up on the banking knowledge from here on out. The Iron Bank and its secret history is more intriguing than you might assume. A lot of people think the Iron Bank is going to become very important in the final episodes of Game of Thrones. Debts are going to be collected, allegiances are going to change, and shady backdoor dealings are bound to happen. That's Game of Thrones for you. Come for the dragons, stay for the banking.
Beware: there are SPOILERS ahead for Game of Thrones.
The Doom of Valyria took place about 400 years before the War of the Five Kings (AKA the main action on GoT) and was said to be absolutely devastating. According to Viserys Targaryen, the Doom wiped out all the dragons and laid waste to the "the Valyrians, their capital city and their surrounding lands."
The Iron Bank was founded before the Doom, back when Braavos was still a "secret city." Diehard GoT fans know Braavos started out as something of a sanctuary to "runaways and refugees from the Valyrian slave lords." The Iron Bank was founded by a group of savvy tradesmen, who decided to begin loaning money out to encourage growth and gain profit.
Cersei certainly seems to think the Iron Bank is involved in the slave trade, if her conversation with Tycho Nestoris in Season 7, episode 4 is anything to go by. Cersei infers that the Iron Bank backing Daenerys Targaryen would be a terrible decision on their part, as the Mother of Dragons has been freeing slaves across the Narrow Sea.
Viewers never get a concrete answer out of Tycho regarding the Iron Bank's investment in the slave trade. He's suitably vague, noting that, "The slave trade has entered a downturn, it's true."
The Iron Bank being a slave financier would be somewhat surprising, considering Braavos's proud claims of being a "free city" and a refuge for escaped slaves. But times change and money never sleeps.
The tradesmen that founded the Iron Bank needed a safe place to store their gold from every opportunistic thief that might be roaming Braavos. They found an abandoned iron mine just outside the city, stored all their riches in there, and sealed it up with gates and chains. They also placed guards at the entrance, each representing a different investor. The Iron Bankers didn’t trust anyone in Braavos, but they also didn't trust each other.
Eventually, all of this distrust dissipated, and the Iron Bank's investors focused their energy on becoming a huge financial titan. They've moved on to grander headquarters, but they still keep the iron bank "not merely to remind us of our roots, but because we did not become the world's greatest depository of wealth by wasting assets, no matter how minor."
Ser Davos and Shireen covered the subject of the Iron Bank during one of their reading lessons in Season 4. Shireen had Davos read from a book about Braavos, and that prompted him to recall his smuggling days. Davos attempted to take a ship transporting gold for the Iron Bank, and nearly got beheaded by the First Sword of Braavos in the process.