As well as the in-game currency of Mirian, there is Middle-earth: Shadow of War Gold running alongside as a premium currency that can be used to purchase loot chests. These stand out from the standard chests you can buy with Mirian, as those you spend Gold on provide a higher chance of receiving better Orcs and items, which means securing a steady supply of Gold can be a real help to your progress and general success in battle.
Helpfully there are a few different ways to get Middle-earth: Shadow of War Gold, both for those that are ready to spend real money, and those that want to earn it without paying another dime.
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Get Gold through the market
Sadly, with any game that has a premium currency, the easiest way to get it is to pay real-life money. Currently the exchange rate for Gold vs. US Dollars is 1 Gold = $0.01, with a bonus of additional Gold granted for purchasing larger packs. In the UK it's slightly less, running at 1 Gold = £0.008, and again you receive additional bonus Gold when buying in larger quantities.
Get Gold through in-game challenges
Fortunately, for those that want to get a chance at better loot chests, but don't want to break the bank, you can earn small amounts of Gold by completing challenges. The challenges change each day, and vary quite a bit in objective, but are never too difficult to complete. Today, one of my challenges was to recruit two Orc Captains in Minas Morgul for 50 Gold. There are also community challenges you can work towards that reward Gold, if you can help complete the group goals.
Do I need Gold to beat Shadow of War?
Premium currency conjures terrible memories of mobile games with massive paywalls, so if you're a bit antsy about Shadow of War's Gold system, it's understandable. However, I never found myself wanting for good gear or powerful Orcs during the game. You get new weapons, armor, and rings thrown at your character pretty regularly, and all of that can be sold for Mirian to buy the non-premium loot boxes if you so desire.
The Orcs and gear in the loot chests that can be bought with Gold in Shadow of War are definitely powerful, and if you're looking for a particular legendary piece of gear or an Orc with specific top-tier characteristics, they can take the frustration out of having to seek them out in-game. However, I haven't seen anything come out of the Gold and Mithril Chests that couldn't be earned in-game, and since everything is tied to Talion's current level, anything you get out of any loot chest is only useful for so long. In short, Middle-earth: Shadow of War is enjoyable without the need for even a single piece of Gold.