All hail the mighty armies of Caesars Legion
Note that this applies only to transforming double-faced cards, not to modal double-faced cards that can normally be played using either face
It's one thing to make a card that enables any card to be played as an instant
Chameleon, Master of Disguise Halfdane Impossible Man Irma, Part-Time Mutant Koh, the Face Stealer Lazav, Dimir Mastermind Lazav, Familiar Stranger Lazav, Wearer of Faces Lazav, the Multifarious Moritte of the Frost Morophon, the Boundless Muddle, the Ever-Changing Omo, Queen of Vesuva Orvar, the All-Form Osgood, Operation Double Super-Skrull The Ever-Changing 'Dane The Prismatic Piper Vazin, Two-Faced Trickster Volrath the Fallen Volrath, the Shapestealer
Spirit of the Night is a nine mana 6/5 with flying, trample, protection from Black, ye olde haste, and first strike when its attacking
and Treasure Mountain to offer cards that could both be the top of the meta and be worth a pretty penny in their enchanted forms