What's the best tributing method? How do I tell what mons have been used for triubtes in the past?
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Cards that have been tributed have a little "x" mark at the top, meaning they can't be tributed or upgraded any further.
And cards that have received a tribute have little heart icons at the top. The more tributes a card has gathered, the higher the chance the next one can fail, but the chance is still always relatively low.