Demonic follower

Hexproof
When this enters the battlefield attach this to target creature. At the begging of your upkeep
you may pay {b}. If you do attach this to target demon. Whenever you cast a demon
you may pay {b}{b}, if you do attach this creature to target demon it gains +X +X where
X is half of it's converted mana cost rounded up
if you do you may pay an additional {b}{b} if you
do this card gains +X +0 where X is the creature this is attached to mana cost.