Amplifying Tome

Arsenal(You may cast this card from your sideboard.)
Armed creature gets an AP counter as long as
it is armed with this card.
Arm {1} ({1}: Attach to target legendary creature you
control as a sorcery. Whenever a legendary creature
you control enters the battlefield, you may arm that
creature as an instant. Until arm resolves, players
can't cast spells or activate abilities that aren't arm or
mana abilities.)