In Assassin's Creed Valhalla, there are four secret weapons made by the mysterious Isu that make for potent additions to Eivor's arsenal. In the series' lore, the Isu were a technologically advanced humanoid race that thrived thousands of years before the first known human civilizations. Though largely wiped out, some Isu still linger on throughout the Assassin's Creed games. More importantly, their works survived in the form of the Pieces of Eden and other notable items containing their power.
Source：https://www.cbr.com/assassins-creed-valhalla-isu-weapons/
