Creatures can't attack you unless their controller pays for each creature they control that's attacking you.
"You've failed Gerrard. You've failed the Legacy. You've failed yourself. I can do no more."
—Volrath, to Karn
(04/10/2004)
The payment is made during the declare attackers step at the same time you are declaring the attacker.
(01/02/2007)
In the Two-Headed Giant format, you still only have to pay once per creature.
(01/02/2014)
Unless some effect explicitly says otherwise, a creature that can't attack you can still attack a planeswalker you control.
(04/10/2004)
If there are multiple attacks in a turn, then you have to pay for each attack.
(04/10/2004)
If there is more than one Propaganda on the battlefield, the cost is cumulative.
(04/10/2004)
Paying this cost is not an instant or any other kind of ability, it is an additional cost on the declaration of the attacker.
