Mei, Herrscher of Thunder

Prowess, Haste
Whenever you cast an instant or sorcery spell, if
it deals damage to a creature or player, it deals
that much plus X damage where X is the number
of spells you have cast this turn from your hand.
"If saving you is a sin, I'll gladly become a sinner." -
Mei Raiden to Kiana Kaslana