Titanosaurus, Sonic Bane

Companion — Your starting deck must contain
at least thirty-five instants or sorceries.
Whenever you cast your second instant or sorcery
on any player's turn, exile the top card of your
library. If it's an instant or sorcery, you may cast
it this turn without paying its mana cost.
Otherwise, Titanosaurus deals damage equal to
the mana cost of that spell to any target.