The language on the card is strange to me. It reads as an action that can be used at any time but the text makes reference to a 'participating character' as if it was a challenge action. I couldn't find a thread about this and wanted to know whether it is actually supposed to be a challenge action. Card text below.
Action: Kneel a Dragon character or Daenerys Targaryen to choose a participating character. Until the end of the phase, that character gets -4 STR and is killed if its STR is 0.