Beastcaller Monolith

Hexproof
If a green spell that has {g}{g} in its cost
will be cast, you may {t} and add {g}{g} to your
mana pool before playing it. If it's a creature
spell, add {g}{g}{g} instead.
{t}: Add {g} to your mana pool.
Monoliths such as these dot the deepest forests of
Unaria, imbuing them with hidden magical power.