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This is quite the powerful effect in a singleton format this can be basically whatever you need it to be, but usually, its a combo piece like Underworld Breach or even a Thassa's Oracle if youve got the Demonic Consultation to go with it

These will transform after you fulfill some kind of condition on the card, usually either an activated ability (i.e., paying to exile Terra, Magical Adept) or triggered ability (i.e., having six permanents in the yard for Exdeath, Void Warlock at end of turn)

Bloodline KeeperBloodline Keeper can repeatedly churn out useful Vampire tokens

Of the three Unown from Lost Thunder that reintroduced alternative win conditions to the game, Unown HAND has been by far the most successful, giving stall decks like Hoopa / Regigigas / Shuckle-GX in Standard a way to win without waiting for the opponent to deck out