For this guide, Im picking Elenda, Saint of Dusk, a black and white legendary Vampire Knight that has a bunch of nifty abilities, not least of which is hexproof from instants
It saves our Angels from removal, blinks Knight of the White Orchid to get more lands (and potentially trigger Emeria Shepherd), blinks Thraben Inspector to generate more Clues, gains us life by blinking Linvala, the Preserver, helps us stay alive by blinking opposing attackers, and probably a million other things I'm forgetting about at the moment
Now that X is in their hand, they play with their hand revealed and they're not allowed to cast X or activate any of X's abilitiesbut you are
So, although you can play her on turn one, you'll still have to wait a turn to have her sing a song