Entrancing Melody
Oracle Text
Gain control of target creature with converted mana cost X.
Card Rulings
9/29/2017 If a permanent has in its mana cost, X is considered to be 0.
9/29/2017 The control-change effect of Entrancing Melody lasts indefinitely. It doesn’t wear off during the cleanup step.
9/29/2017 In a multiplayer game, if a player leaves the game, all cards that player owns leave as well, and any effects that give the player control of permanents immediately end.
Enough cards like this and you could legalize Dark Depths for modern.
I mean you could anyway, but whatevs.