UPDATE: Thanks to feedback from fans, the period players have to unlock the Acrophobia Skull in Halo 3 has been extended by “at least” 30 days, according to 343 Industries community manager Brian Jarrad. A longer-term fix to allow players to unlock this skull in the future seems to be in the works, too.

Still, if you want this skull, it’s perhaps best not to dawdle–start taking down flying enemies now.

The original story follows.

Halo 3 recently came to PC for the first time through the Master Chief Collection, and it brought with it some upgrades and change. One temporary addition is a new unlockable skull that adds an incredible new power to the game–but there’s only two days left to unlock it.

The Acrophobia Skull lets Master Chief fly his way through the campaign, regardless of whether he has access to a Ghost or Banshee. The skull lets you fly by holding down the jump button, rising perpetually, taking to the skies to see each level of the campaign from a new perspective. The skull can also be used inside other games in the Master Chief Collection.

The skull can be used in single-player or co-op, but you’ll need to work fast and hard to unlock it. You’ll need to defeat 343 flying enemies (such as Banshees, Buggers, Phantoms, or Sentinels) before the end of July 29, 2020. You’ll need to be playing on Normal or harder.

It’s the kind of old-school cheat that games so rarely have these days, and it puts a fun new spin on the game, so you don’t want to miss it. The best way to do it is to play through a mission will a lot of Bugger enemies–like The Storm–and reload checkpoints when possible. It’ll take a while, but it’s worth doing.

If you’re enjoying Halo 3’s return to the spotlight as much as we are, check out editor Eddie Makuch’s story of how the game changed the lives of its own developers.