The Stars Align

Cascade
You may cast an instant or sorcery from
your hand with mana value X or less, where
X is the number of cards exiled this turn. If
X is 10 or more, it gains cascade.
—
"The stars themselves are pools of mana. Mana
yet untouched. Pure, and powerful, there truly is nothing like them." - Giovanna, Spatiumancer