[mwcard=MW1E30]Pacify[/mwcard]
The payment of 2 mana is part of the cost of making the attack. Thus, you cannot declare the attack if you don't have the 2 mana to pay.
You cannot reveal Pacify “in response” to the declaration of an attack in order to force the opponent to pay 2 extra mana. Pacify can be revealed before the Declare Attack Step, or after it, but not during it. (Once an attack is declared, the earliest opportunity you could reveal Pacify is at the end of the Declare Attack Step, which is after costs are calculated and paid.)
Pacify will work against spell attacks. On a spell attack, since the attack is actually made when the spell resolves, the additional 2 mana is paid when the spell resolves. (For example, if a Familiar is casting an attack spell, his opponent could reveal a Pacify at the end of the Counter Spell Step. During the Resolve Spell Step, the spell would resolve and the attack sequence begins, at which point the Familiar’s controller would have to pay 2 mana if able; if not, then the attack would not happen and the spell would be wasted.)
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Somehow i cannot follow this.
If a Creature declares a melee attack, it's too late to reveal Pacify, because the Declare Attack Step has passed.
What is different with a spell from a familiar? The first step when casting a spell is a declaration step too. Why would revealing [mwcard=MW1E30]Pacify[/mwcard] after this step do any good? The Resolve Spell step is obviously after the Declare attack step.