I met a new question. When I play Lannister House, and use Varys's effect to change plot cards with Targaryen House plot card. Now, Targaryen House just have last plot card is Mad King's Legacy (T205), but this card is House Targaryen Only. how to deal with this situation? Can Lannister reveal and trig this plot card or not? If not, can Lannister take back used plot card?

"House X only" is building deck restriction. You cannot build a deck with cards House X only unless X is Your House Card. Only this:)

So Varys can play like this:)

Said another way, "House X only" dictates that the card can only be brought to the game in a deck with the corresponding House card. The owner of the card must be playing that House. But once the game starts, "owns" and "controls" are not necessarily the same thing. So "House X only" has absolutely no impact on how the card acts once the game starts.

Thanks a lot, I see.