Play up to 1 Prisoner of Impel Down card from your hand." The deck isn't just going wide, it wants its own cheap prisoners to trade off in combat so the leader ability refunds a card back onto the board for free
But it's also true that people can use Encroaching Mycosynth to make the battle an artifact, then The Blackstaff of Waterdeep to make it a creature, then Arcane Adaptation to make it a Human, then Moonmist to transform it, and because of the changes to DFC token rules introduced for Incubator tokens, that token would then transform
The Monarchs Stormforth X SPELL Quick-Play Once during this turn, if you would Tribute a monster for a Tribute Summon, you can Tribute 1 monster your opponent controls even though you do not control it
1) as an Equip Spell that gives this card 500 ATK
You can cast them more efficiently than other spells of its type, and get to the ability that comes with it faster
If the enchanted creature is no longer on the battlefield when Penumbra Umbra's second ability resolves, use its copiable values as it last existed on the battlefield