Mephistopheles the Abhorrent

You can't cast this spellunless your current life
total is less than half of your beginning life total.
Eminence—if you would cast a Demon creature
spell, instead of paying it's Mana cost, you may
pay life equal to twice it's converted Mana cost.
If the effect of a Demon you control would
cause you to lose life, you may gain that much
life instead.