Farahl, Herald of Blight

Wither
Whenever a creature you control would deal
damage to another creature with toughness
greater than that creature's power, you may pay {x}, where X is equal to the difference. If you do,
that creature gains +X/+0 until end of turn.
"Xiin-Clieva left us here to rot on this twisted plane.
Yet no matter how many loops I experience, my anger
shall fester on forever."