A player loses the game after having been dealt 21 combat damage from any one of them, not from both of them combined (although your Background wont usually be a creature anyway)
A lot of black cards will emphasise the sacrifice element which will come in the form of direct downsides to your cards such as you having to lose a creature or pay life to achieve something
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Secret Lair Final Fantasy: Game Over Final Fantasy Secret Lair MTG drop focuses on iconic, game-changing moments
Mari, the Killing Quill There are other assassins that add hit counters, but Mari, the Killing Quill can remove them to get you Treasure
When The One Ring enters, if you cast it, you gain protection from everything until your next turn