@Red Queen - a simple persuasion check when speaking to the fallen knight “convincingly explaining your reason for your actions and categorically insisting that your oath is still strong” should be enough. It’s at an appropriate time (at camp) and will not break the flow of the game. Depending on what you did could determine the save you would need. IE stealing the Idol or killing a Tiefling 25. Letting Astrrion live maybe a 5 depending on sub class…
