Helm of Nabu

Equip {4}. Equip only as a sorcery.
Equipped creature is Mystical and has flying.
Equipped creature has protection from instant and sorcery spells
Whenever you play an instant or sorcery spell, put a charge counter on Helm of Nabu.
Equipped creature gets +1/+1 for every charge counter on Helm of Nabu.
If Helm of Nabu becomes unequipped, put the creature it was equipped to into its owner's graveyard.