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CardName: Screaming Apparition Cost: 2B Type: Creature - Spirit Pow/Tgh: 2/3 Rules Text: Defender At the beginning of your upkeep, Screaming Apparition unsettles each opponent. (--reminder text missing--) Flavour Text: Set/Rarity: [Assorted] Card Repository Common |
Code: History: [-] |

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Discard would be my first instinct but that can't be it since it would be ill-advised for a mechanic (too much CA) and this is a common.
Maybe "exiles the top (X?) card(s) of their library"?
Others I could see would be somehow stop or make attacking or blocking harder. That doesn't seem particularly
though...
My first thought was:
...unsettles each opponent. (That opponent can't cast instant or sorcery spells this turn.)
...but it's a little too
to be a 
mechanic. So the other idea is:
...unsettles each opponent. (They can't target creatures attacking them with spells or abilities this turn.)
It's a bit of a bend, but it'd be found in

and maybe
, as a "you're so nervous you can't aim straight" type of deal.
For a joke mechanic:
...unsettles each opponent. (Each opponent shuffles his or her hand, searches his or her graveyard, then says "Resolves.")