A Spectral Panther, if summoned by one without foresight, is little more than a paper tiger.
This beast NEEDS hand/deck buffs like Prince Keleseth, Smuggler's Crate, or Grimestreet Outfitter in order to be summonable, because if it doesn't have at least 1 Attack when you play it... It enters the field with zero health. But if you manage to complete the (honestly not so difficult) setup, then you have a better Sabretooth Stalker on your hands.
This is a truly devious card that supports the inevitable Treachery Warlock. As the flavor text suggests, you can force an enemy minion to become a harbinger of impending doom. So why is this priced at 3? Well, the effect is random, and even a Treachery Warlock can't run only dangerous minions. Also, the stats aren't swapped, meaning that though you can save mana by not playing the minion, you're now dependent on your opponent's body. Still- awesome power (Hydra) that can backfire horrifically (Lich King, Grim Patron, Ragnaros)? Sounds like a Warlock's cup of fel.