Roiling Horror's power and toughness are each equal to your life total minus the life total of an opponent with the most life.
Suspend X—{X}{B}{B}{B}. X can't be 0. (Rather than cast this card from your hand, you may pay {X}{B}{B}{B} and exile it with X time counters on it. At the beginning of your upkeep, remove a time counter. When the last is removed, cast it without paying its mana cost. It has haste.)
Whenever a time counter is removed from Roiling Horror while it's exiled, target player loses 1 life and you gain 1 life.