This assassination occurs when Eivor travels to Vinland, which I've learned is currently Newfoundland and the Gulf of Saint Lawrence in Canada. Eivor is posing as a slave and doesn't have the usual arsenal of weapons and armor. The target Gorm Kjotvesson is located at one of three camps where he directs a digging operation. This camp is more heavily guarded than the other two.
I went inland in order to approach the camp from above. Coincidentally, this starting point turned out to be exactly where Gorm goes after giving a speech to his men. My goals for this assassination were:
The first step is to approach Gorm. This triggers him to begin his speech. It turns out that he has a Piece of Eden but hasn't figured out how to work it yet. When he shakes it and commands it to speak, Juno frustrates him with a line right out of a fortune cookie. Stick around until the end of the speech to improve the odds of Gorm going for a walk afterward. Sometimes he just stands there. I don't know why.
I sneak back to the same cliff where I started. From there it's an easy jump to the roof of the archers' tower. Note that one of Gorm's bodyguards climbs into the tower but doesn't notice Eivor on the roof right over his head. A funny thing happened while recording this video. Gorm came across a dead body and stopped to inspect it. I certainly didn't leave it there and there wasn't an option to Carry it away when I passed over it earlier. Wait until Gorm comes into range and go for the air assassination. He's a high level enemy so you either need to complete the QTE or disable those in the game options. Otherwise he survives the initial attack.
After the Memory Corridor scene (omitted from the video) there's an additional cutscene where Eivor examines the Piece of Eden. You can begin your escape while Eivor's still talking. I escape through the tunnel to demonstrate where to find it. Some players infiltrate the camp through this tunnel because the other end by the water is marked as a Secret Entrance on the map. There's an escape canoe conveniently parked near that end.