Electro, Assaulting Battery (SPM 76)

Legendary Creature — Human Villain • Mana cost: {1}{R}{R} • Marvel's Spider-Man • Rare

Flying
You don't lose unspent red mana as steps and phases end.
Whenever you cast an instant or sorcery spell, add {R}.
When Electro leaves the battlefield, you may pay {X}. When you do, he deals X damage to target player.

Illustrated by Piotr Dura.