Warning: SPOILERS for the Game of Thrones season 7 finale ahead!
As expected, the Game of Thrones season 7 finale had some major twists and turns. However, what should have come as no surprise was Cersei publicly agreeing to do one thing, while secretly scheming to do something else entirely.
After witnessing the terrifying wight brought back from the north and being negotiated with by Tyrion, Cersei seemingly agrees to not only a truce but to lend her forces to those of Daenerys and Jon in order to fight the Night King. Except, that's just what she says she'll do. What Cersei really intends to do is let the Stark and Targaryen armies either be defeated by the Night King or wear themselves out fighting him, leaving her armies to clean up whatever's left.
Of course, the Lannister army is pretty well decimated thanks to Drogon, but Cersei has seen to this, using the gold taken from Highgarden (and possibly another loan from the Iron Bank) to hire the greatest mercenary army in the world -- The Golden Company of Essos. Who are they and why is their hiring so significant? Let's break it down.
The Golden Company of Essos
Like Daario Naharis' Seconds Sons or the Stormcrows hired by Ser Davos to serve Stannis, The Golden Company are a group of sellswords or mercenaries (though they prefer being called a brotherhood of exiles). For a price, they'll ally their armies with yours and help you win your next battle. Most sellswords are notoriously disloyal and if your opponent pays a larger sum, they'll switch sides almost instantly. The Golden Company, however, has a reputation for never breaking a contract and are ruthless with those who break with them. Their motto is "Our word is as good as gold," and being the largest and most famous of the sellsword companies that operate in the Disputed Lands (a region of Essos which the Free Cities constantly war over), they are also the most expensive.
Founded by one of Aegon IV Targaryen's legitimized bastards, Aegon Rivers (AKA "Bittersteel"), The Golden Company came about in the aftermath of the First Blackfyre Rebellion -- during which Aegon IV's bastard son, Daemon Blackfyre tried to usurp the throne from Aegon's trueborn son, Daeron II -- and originally consisted of those exiled lords and knights who had backed Blackfyre. That trend of enlisting exiles from Westeros continues even today, for example, Jorah Mormont, himself a Westerosi exile, took up with The Golden Company when he first came to Essos before swearing allegiance to Viserys, and later, Daenerys Targaryen.
Under Bittersteel and his various heirs, The Golden Company became an immensely disciplined army, able to quickly assemble and march on the enemy after even the most chaotic of landings. In addition to infantry and mounted knights, The Golden Company is also said to have a few elephants in their ranks. As an added bonus of intimidation, their officers and generals proudly display their wealth in ornate swords, fine silks, or with gilded skulls displayed above their banners, and it's said that each member wears an arm ring of gold for every year of service. Hiring The Golden Company is an expensive endeavor, but it's a move that can practically guarantee victory -- assuming, of course, you don't don't double cross them or refuse to pay.