There are three prospective Assassin's Creed Valhalla villains that no one expected: Grendel, Grendel's mom, and a big-ass dragon.
Assassin's Creed Valhalla will have a side mission based on the events of Beowulf. It's called "The Legend of Beowulf," and it's exclusive to the season pass. (Although, it's seemingly only bonus content, not actually a full-fledged add-on.) This was accidentally posted to the Ubisoft Germany site along with the description "Discover the cruel truth behind the legend."
There are a few different approaches Ubisoft could take here. It could be a retelling of sorts, but with Valhalla protagonist Eivor ripping off arms and cutting off heads. Or, it could be a postmortem with Eivor retracing the steps of Beowulf. Assassin's Creed hasn't shied away from mythology in recent years, with Origins DLC sending Bayek to battle Egyptian gods and Odyssey putting Alexios/Kassandra up against Greek monsters.
This is actually Ubisoft's second stab at Beowulf. Back in 2007, Ubisoft developed the Beowulf video game that accompanied the film's release. There's no word as to whether Angelina Jolie is expected to reprise her role as Grendel's MILB (Mom I'd Like to Behead).