Reach level 60 and complete the campaign with a character to unlock alliances in World of Warcraft Shadowlands.
Alliances are an endgame feature in World of Warcraft Shadowlands, so before you can swear allegiance to any of them, you'll first need to complete the Upgrade Campaign and reach level 60 on the one of your characters. You have to overcome both of these hurdles, so even if you're 60 long before the final act (like I did), you'll still need to cross the storyline finish line before you access Alliances.
Once you see the end of the Revendreth arc and are redirected to the Oribos hub, you will quickly resume the "Choose Your Objective" quest. This is your big moment, your chance to sign one of the four main covenants that rule the afterlife in WoW. Each Covenant will have a representative in the Enclave that you can interact with, and you will have several options. The first choice provides a general preview pane that will display the two abilities offered by the Covenant (with tooltips), the three available Soulbinds, the Covenant's Sanctum Feature (a Covenant-specific activity), their main armor (for your class), and their mount.
You can then choose to sample the Covenant's abilities and test them out on a nearby dummy if you still don't know what you prefer (or want to analyze which is best). Underneath is the option to reset ability cooldowns for additional testing. You can ask what kind of adventures to expect with this Pact, which provides flavor text and insight into the challenges Covenant faces. And, finally, you can commit to the Covenant. You'll have to entrust the quest to Tal-Inara to cement your choice of Alliance, but once you do, you're not really tied to your choice. You can change alliances at any time by interacting with one of the representatives who will remain in the Enclave, but you will need to complete two weeklies (one per week over two weeks) to revert to the alliances you left behind.
From now on, your alts can choose a Covenant immediately after escaping the Maw for the first time, assuming you choose the Threads of Fate option. Interestingly, Threads of Fate is a permanent choice, but you can join at any point during the campaign (unless you hit level 60). Anytime you choose Threads - whether at the start of or during your alternate journey - you can then choose your alliance. You will speak with a representative in the Enclave and make a choice as you did earlier with your first character. I recommend sampling the abilities of the class first; each Covenant has an ability specific to their class, so it doesn't hurt to see if you like them.
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