The Halo Infinite weekly ultimate challenge pops up just when you thought you'd finished with the challenges. It's the last step toward getting the week's unique reward, though admittedly, what you get for the effort is often not equal to the work it took to get there.
Most weeks, you'll get a new emblem or something for your banner, though a few more inspiring awards have shown up from time to time. Just plan for the long haul if you want them. They're called "Ultimate" challenges for a reason.
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How to Unlock Halo Infinite Weekly Ultimate Challenge
Halo Infinite weekly ultimate challenges only open once you've cleared all the other weekly challenges, hence the name. You get 20 of those every week, so it'll take a while - though it shouldn't be too annoying now 343 has removed more of the mode-specific challenges. If you get one you absolutely can't complete, though, it's worth using a Challenge Swap to knock it out and get something more feasible.
What Is This Week's Halo Infinite Weekly Ultimate Challenge? (January 12)
This week's weekly ultimate challenge is named "Buddy System". In order to complete it, you have to earn 3 double kills in PvP modes.
As always, progress made before the weekly ultimate challenge unlocks doesn't count towards the challenge. In other words, you have to get the double kills after unlocking the challenge, on top of however many you managed to get during the regular weekly challenges.
What Is This Week's Halo Infinite Weekly Ultimate Challenge Reward? (January 12)
Once you've cleared all 20 weekly challenges and completed the Ultimate challenge, you'll earn the Sacrifice Armor Emblem.