Instead, you'll have to use the Gold premium currency to purchase them. These two chests can't be bought with Mirian though. Gold War Chests will give at least one Legendary and two Epic Orcs while the Mithril War Chest will provide you with four Legendaries every time. The only way to guarantee a Legendary Orc Follower is by buying the Gold and Mithril War Chests. Of course, there is the possibility you can get a Legendary Orc in a Silver War Chest, but it's random.
For 1,500 Mirian you can get a Silver War Chest, which guarantees at least one Epic Orc and an additional Orc of standard level or higher. By using Shadow of War's marketplace you can be sure that you'll get Epic and Legendary Orcs, but you'll have to pay for it.
