Equilibrium | Magic: The Gathering
Formats Equilibrium is Legal in
Official Oracle Text for Equilibrium
Official Rulings for Equilibrium
8/1/2008 : The ability is put on the stack immediately after you cast a creature spell, which means the creature that spell will become won't be a legal target.
8/1/2008 : The ability will still resolve even if the spell that triggered it gets countered.