Best posts made by SciPhiFan
For those of you that may still be having a problem completing this quest, maybe what I finally did will help you.
I play on XBOX ONE.
I spent over an hour searching the internet and trying different variations but finally came across one that really helped. That being the case, I feel I should give credit to "JoeTheBard" who authored the following: https://www.gosunoob.com/assassins-creed-valhalla/lost-drengr-ragnar-lothbrok-reconstruct-ragnars-death/ I hope this is OK to do this. Thank you JoeTheBard.
In his description, he mentions "tap R3, rather than hold it as usual". On the XBOX, R3 = Right Stick. Normally, Holding the Right Stick gives you FOCUS and Tapping the Right Stick on the XBOX LOCKS you on an enemy target, so that locked me on one of the snakes. So I killed all of the snakes, even though other guides I found said not to. Once the snakes were dead, I looked at the marker on the cage and tapped the Right Stick quickly and the cutscene activated.
I hope this helps.