Lucifer the Reckless

When Lucifer the Reckless enters the battlefield,
it fights target creature.
Whenever you cast your first spell on an opponents
turn, Lucifer the Reckless gains +X/+0 where X
is the spells converted mana cost and fights
target creature.
At the beginning of your upkeep, if Lucifer the Reckless didn't fight last round, sacrifice him.