I'm not able to find out from rules and FAQ how treachery works in Road to Legend. I see that overlord buys it for XP, but when it comes to spending it is not clear for me if Overload buys some cards for treachery and they are added to his deck and are available in all subsequent dungeon/rumor/etc quests or these cards are bought only for one quest and after that quest are discarded. Alternatively I can see the third variant that having some treachery bought overlord can spent it before each quest to buy different cards and they are discarded after the quest.
So my questions are:
1. Can treachery be spent several times before different quest or only once?
2. Are cards bought for treachery discarded from overlord deck after quest ends?