Symbiotic Mutation

At the beginning of your upkeep, you may lower any one creatures power and toughness, and add that amount onto one other creature. You cannot lower a creatures power or toughness to less than zero. If a creature dies from this effect, you take 4 damage.
{5}{u}{g}: Target creature has its power and toughness doubled. Destroy it in your end phase. Only activate this ability in your combat phase.