The war between the two queens is heating up (literally) on Game of Thrones. After the Loot Train Battle in "The Spoils of War," Cersei is dealing with her first major defeat, but it may not be a total failure for the Mad Queen in training. If Randyll Tarly didn't misspeak, then the gold the Lannister army takes from Highgarden is already safely within the walls of King's Landing. Assuming Tarly is right, then Cersei just got the influx of gold she needs to pay her debts and borrow enough money from the Iron Bank to hire the Golden Company. What is the Golden Company, you ask? They are a sellsword company said to be 10,000 strong. Based in Essos, this band of exiles is known for never breaking a contract. Cersei wants to use the money she plans on borrowing from the Iron Bank to hire out these men and bring them to Westeros in order to replenish her dwindling army.
Now that the Lannisters have lost even more soldiers, the Golden Company could be the key to keeping the devious twins in the game. In the books, the group of sellswords is made up of highly trained men willing to fight for anyone with enough money to pay them, and they take their job seriously. While sellswords in general are thought to be oathbreakers by nature (that's why Stannis declines to hire the company when Ser Davos suggests it in season four), the Golden Company takes great pride in their reputation. As a result, they keep their word once money exchanges hands. But just because they've never broken a contract before doesn't mean they never will, especially when it comes time to fight for Cersei.
The Lannister queen is making a smart move by seeking out the Golden Company, there's no doubt about that. They are a ready-made army of trained men with battle experience. And since they are based in the Free Cities, they may have experience fighting against Dothraki Screamers, something the young soldiers of Westeros are clearly not prepared to do. One potential downside is that she could be seen as bringing a foreign threat into the land, just like she accuses Daenerys of doing, but after the Loot Train Battle, her remaining allies will surely prefer the Golden Company's services over losing more of their own men to dragon fire. As a group, these sellswords could give Cersei major leverage, but there's one thing the queen isn't taking into consideration: the Golden Company could be swayed to Daenerys's side.
Targaryen bastard Aegor Rivers is the founder of the Golden Company. Many years before the events of Game of Thrones, he leaves Westeros and gathers the exiled knights and lords seeking sanctuary in the Free Cities to form a company of sellswords who stay true to their word. Even though this current iteration of the group never met Aegor, their company would not exist if not for a Targaryen. This could work in Daenerys's favor, especially when Jorah is factored in.
Presumably, the newly cured Jorah is making his way back to Daenerys, and his timing couldn't be better. Way back in season four, the Dragon Queen chastises Jorah for being suspicious of Daario's sellsword past because Jorah himself once worked for the Golden Company. That's right; before he is enlisted to spy on Daenerys and later switches to her side, Jorah worked for the most famous group of sellswords in the known world. Whether or not the knight left on good terms remains to be seen, but his passion for his queen could compel him to intercede on her behalf with his old band of brothers.
A dragon is responsible for founding the Golden Company, and even gold may not be enough to make them fight against the last Targaryen in the world (at least, the last one anyone knows about). Granted, their entire reputation is built on not breaking their contracts, but in the books, they compromise their motto to fight alongside a Targaryen who isn't a part of the show — Rhaegar's supposed son, Aegon. Don't put it past the Game of Thrones writers to re-create this story on the show with Daenerys standing in for Aegon.
Whether the Golden Company fights for Cersei or Daenerys, look for this crew of sellswords to turn the tide of the war for whichever woman is lucky enough to contract them. A ready-made army with 10,000 fighters is bound to be formidable, whether they're fighting alongside dragons or against them.