The footage comes from the perspective of Eivor, the female lead in Assassin's Creed Valhalla. It starts with Eivor riding her horse through a village, only to be attacked by wolves as she exits town. Eventually, Eivor meets with a man named Finnr and they discuss their gathering army and an assault that they're planning on Burgh Castle and a man named Rued. Rued's clan apparently controls the region of East Anglia and isn't a particularly kind leader.
Next, Eivor is part of an assault on Burgh Castle. A sequence where the longboats of Eivor's warriors move toward the keep while flaming arrows rain down is particularly striking. Then, after an explosive boat takes down one of the keep's walls, Eivor leads the siege into the keep's walls. From there, she takes down foes with her axes, helps use a battering ram to take down gates, and pushes further and further in.
Rued is eventually defeated, his fate to be decided by the player. From there, the player takes some time to look through Eivor's skills and armor, setting her up with a new look. And the final minutes of the video are spent looting the newly freed Burgh Castle and exploring.
It should be clarified that the footage is marked clearly with a "Work in Progress" label and that there are some notable polish issues left for Ubisoft to clean up. Ubisoft still has a good amount of time before Assassin's Creed Valhalla launches, so it's nothing to worry about.
It's unclear where the leaked footage is sourced from, but it seems unlikely that it was ever intended to be shared publically. Publishers prefer in-development game footage to be carefully curated. Hopefully, those who do end up seeing the footage are understanding.
Assassin's Creed Valhalla releases later this year on PC, PS4, PS5, Stadia, Xbox One, and Xbox Series X.