Windborn Muse

Oracle Text

Flying

Creatures can't attack you unless their controller pays 2 for each creature he or she controls that's attacking you.

Card Rulings

10/4/2004 If you have more than one of these on the battlefield, the cost is cumulative.
2/1/2007 In the Two-Headed Giant format, you still only have to pay once per creature.
2/1/2014 Unless some effect explicitly says otherwise, a creature that can’t attack you can still attack a planeswalker you control.