It seems like if you have a settlement garrison hero become your challenger from the Living Legend mechanic, and you occupy/sack/raze the settlement without Achilles in one of the armies involved in the battle you will become stuck with a challenger that you can never remove, and therefore can never resolve the status of Doubt and Uncertainty.
Looking at the auto save the turn I received new challengers.
Garrison showing that the challenger is in the settlment's garrison.
I sacked the city, and now there is a string dy_flavour_text listed where Awe, Intimidated, Stout, Etc. variable would be listed.
Here I clicked confront. Now this button is permanently greyed out.
Even if you destroy the faction this challenger came from, this will persist on the screen and forever prevent you from completing it.
If Achilles is the army that Occupies/Sacks/Razes or is in a reinforcing army this will complete the challenge however.