I find it horrifying if one had the power to control death itself. They could grow stronger the more they kill or when people die around them. In extreme cases, they could make corpses their undead minions. If they had that level of power, they could unleash an unstoppable zombie outbreak to annihilate their enemies or conquer nations. To put it in general terms, the wielder of this power would be the incarnation of the grim reaper and the Horseman of Death!