This article was updated on January 28 after the discovery of the proper way to unlock Assassin's Creed Valhalla's secret Isu bow. If you want to do it without using any tricks (aside from the ones devised by Ubisoft) then head here.
Players of Assassin's Creed Valhalla have found a super-powerful magic bow hidden in a pile of shiny rocks, and they've found it in a way that the developers absolutely did not intend. The secret bow, created by Assassin's Creed's alien space gods, the Isu, is located underneath a pile of rocks. There's some secret path that nobody has quite found yet, but there's also the way where you find the pile of rocks and hit it some and then reload your save and then hit it some more. Here's AC Valhalla Narrative Director Darby McDevitt on the hack:
FYI this is a clever brute force hack, not the "proper” way to get this bow. There is an “in world” way to achieve it. But I suppose this works for an Any% Speedrun. 😎January 24, 2021
The weapon itself is an extremely powerful bow made by the ancient Isu, which is part of the lore of Assassin's Creed that only people who really really care about Assassin's Creed know about. The short version, at least, is that they're godlike aliens that used to rule humanity. So... it's an alien space bow. It's called Nodens' Arc, a reference to the ancient Celtic god associated with things like hunting, healing, and dogs, and equated by the Romans with their god Mars.
The video below, from video creator JorRaptor, explains how the exploit works.
How to find the secret Isu bow the regular way
If you'd rather earn your fancy magic bow than brute forcing your way to it by hitting a pile of rocks a bunch and reloading, the actual solution has now been discovered.
AccessTheAnimus has conveniently detailed the steps you'll need to take, which involve heading to the rocks and using only the very best of weapons to break them with any tricks. You'll need Excalibur if you want to do it yourself, and you'll need to wait until just the right time. Excalibur isn't a sword you're likely to get until quite late into the game, but you can find out how to do that with our Excalibur guide.
The video above details exactly how it was figured out, so at least you can pretend you were involved in solving this conundrum. A lot of you will probably just be wanting to the solution (skip to 8.52 for that), but I really appreciate being shown what lead to it being discovered.