If you can't comply with the first sentence, you cannot do what is followed by THEN.
i.e. no sacrifice after you can't find (or don't want to find) a Lord/Lady, or if you can't put it into play (barring the gates). This is confirmed by FFG Official.
Was wondering how to use this card efficiently as part of my deck? I mean does it block every power gain even for power claims? Or is it just to stop characters with "renown" keywords from gaining power if they're not Kings/queens? The latter seems to make it useless unless you just want gold considering not a lot of decks in general have tons of renowned cards in the first place. Be great to know if I should leave it in my deck or put it with my "cards that don't do much" piles?
Your King Commands It blocks all non-king/queen characters from gaining power, either from renown or through other passive power gain abilities (e.g. 7-cost Catelyn Stark, Consolidation of Power). It is a good card to shut down certain rush decks, like Tyrell 'knight'-heavy decks, which rely on lots of characters gaining power from renown.
If you use the agenda action to put a card with printed shadows into shadows, it does not gain shadow (X).
If you place a card with cost X into shadows, it gains shadows (0). If it is played from shadows, X equals 0. This means that e.g. Gifts for the Widow could only search for cost 0 attachments if played from Shadows.
The Hand’s Judgment can be played for free to cancel an event from shadows.