I don't know if this will work, since it's borderline but arguable, but here goes:
You cannot win the game. Your opponent cannot lose the game. (Note: this means that there cannot be a draw)
CARDNAME cannot be the target of your own abilities.
When you play CARDNAME, you lose 20 morale.
Behold the dark summoning circle, splashed with blood. Each sacrifice was cut in two, unholy food. Three bodies there were, angels from afar. Their pieces formed a six-pointed star. Fell was the creature that sprang forth. A shadow cast from south to north. The summoners fled, all betrayed. Their spirits broken by the demonic raid. Treacherous Heresiarch. Your villainy ne'er misses its mark.
On the one hand, it doesn't outright win your opponent the game. On the other, they can't lose either. Most stupid combos are negated by making it so you can't target it yourself, it costs a hefty chunk of morale just to play (seriously hefty, to bring home the challenge theme), and the effect of "you can't win, your opponent can't lose" is blatantly lifted from MtG. The ideal combo with this card is to play it, deal some 90 damage to your opponent's fortress, then either Calamity or Mass Death to get rid of it and finish your opponent off with, say, a Siphon Structure. You can also block with it. As an opponent, you can block or stumble it to stay alive whilst dealing as much damage as possible to the controller and theoretically securing the win. If you want it gone for that hefty morale hit, as the opponent you can target it, so those Death Rays and Hubris of the Strongs and Demonizes will all work. It isn't unique because where's the fun in that? If you want to play multiples of this demon, more power to ya.
In short, playing this card gives your opponent the following options: Keep it alive, and you can't lose. Kill it, and the player will lose enough morale that they've probably lost.