Sphere of Safety
Enchantment
Creatures can't attack you or planeswalkers you control unless their controller pays 
for each of those creatures, where X is the number of enchantments you control.
Even as the barrier began to falter, the swarm grew stronger still.
Illustrated by
Secret Lair Drop#2331RareEnglishLegality
Rulings
Oct 1, 2012
If you control Sphere of Safety, your opponents can choose not to attack with a creature with an ability that says it attacks if able.