In short, if you want to activate Bifrost, you will need first to kill at least one Jotun. Once you kill a Jotun, Bifrost will activate in your world, and you will be able to use it to keep all of the golden horns you earn during your playthrough.
However, before this point, you will need to survive to at least day three in the game. When day three hits, you will be able to fight your first Jotun, as it will spawn around this time - it can sometimes be early or later before a Jotun spawns.
In the time before the fight with the Jotun, we highly recommend raising your power level, so you will be able to survive a fight with it and deal enough damage.
You should also bring a few friends along for the ride so you can kill Jotuns faster and get other tasks done faster in the game. That said, once you put the Jotun in it's place, you will get a pop-up telling you Bifrost is activated.
After this, you will then be able to use Birfost to leave your game while also saving any golden horns you earn while playing.
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Andrew Vaughan is a Contributing Writer at Pro Game Guides that has written guides on popular RPGs and FPS survival titles for several years. Andrew Vaughan specializes in survival and roleplaying games and has been a lifelong gamer.