Creatures can't attack you unless their controller pays for each creature they control that's attacking you.
"The cost of your opposition will be extracted from your wits."
—Nekusar, the Mindrazer
(04/10/2004)
If there are multiple attacks in a turn, then you have to pay for each attack.
(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 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.
(01/02/2007)
In the Two-Headed Giant format, you still only have to pay once per creature.
(04/10/2004)
The payment is made during the declare attackers step at the same time you are declaring the attacker.
