Seems like pretty good tech against other classes DK Hero cards, assuming they are classed as minion before being played like Lord Jaraxxus? I mean, they don't look like minions, or spells? Are they just unique?
If that does work depending how popular certain ones become. Being a Burn Mage and stealing Valeera the Hollow is game winning.
Death Knights aren't minions. THey have no stats. Can't pull them with Dirty Rat for example.
Well, sometimes people play around Mirror Entity by playing Battlecry minions, so this at least allows you to use those battlecries. It also provides some added value for Secret mages, which they definitely wanted with Duplicate gone. That said, this card is certainly worse than Duplicate because it's far less controllable.
I figured they wouldn't print Glacial Mysteries without realizing Mages don't have secrets that generate value in the late game. A staple in those decks for sure. Just like with Mirror Entity, you need to play this later in the game to make it harder to give you dud cards. A fine secret.