Novawurm engine

Trample, Hexproof, Vigilance
Whenever Novawurm Engine attacks, you may
pay {x}. If you do, it deals X damage to target
creature or planeswalker defending player
controls, where X is the amount of mana paid
this way.
Forged in the heart of a dying star, the Novawurm
Engine harnesses the very essence of celestial power.