Alexstrasza, the Life-Binder

Flying
When Alexstrasza, the Life-Binder enter the battlefield, you gain X life, where X is the total
power of creatures you control.
{2}{w}{r}{g} : Exile Alexstrasza, the Life-Binder,
then return her to the battlefield transformed
under her owner's control. Activate this ability
only any time you could cast a sorcery.