The sometimes free part comes from its convoke cost, letting you tap four creatures (two of which have to be white) to pay for the casting cost
Plus, in addition to keeping your stuff rocking the roost, we have a 6/6 beater for 6 mana, and the ability lingers in your graveyard like a one-shot espresso to keep the team awake for another hour
Why proxy it: High power level across multiple formats, significant price, and frequently required in multiples for cube builds
Umbris will be bigger each time you cast it, and you easily win via commander damage by making it unblockable, or by using infinite ETB engines to mill opponents out
This is nowhere near the end of how useful this card is, though, and were going to see it in a lot of decks in the future
Cards like Narset, Parter of Veils and Spirit of the Labyrinth can force players into a hellbent situation when paired with a wheel