This mystery can be found in the south of Lincolnscire, at the ruins of Cruwland, north of Medeshamstede Abbey.
Interact with the Throne at Cruwland and Defeat the Bandits
When you reach the ruins of Cruwland, you’ll hear some voices coming from a mysterious source. Enter the ruins and approach the throne to the north. When you interact with the throne a brief cutscene will begin, during which you’ll have a few dialogue options in response to the voices. It doesn’t matter which option you choose here, as the outcome will be the same. The bandits will demand that you get off the throne and leave, and when you don’t they’ll attack you.
The bandit group mostly consists of regular enemy units, but there’s also one Man at Arms. Nothing a seasoned Eivor can’t handle. Once they’re all defeated, the mystery will be complete.