When power word, kill is uttered, the character can either target a single creature or let the spell affect a group.
If power word, kill is targeted at a single creature, that creature dies if it has 100 or fewer hit points.
If power word, kill is cast as an area spell, it kills creatures in a 15-foot-radius sphere. It kills only creatures who have 20 or fewer hit points, and only up to a total of 200 hit points of such creatures. The spell affects creatures with the lowest hit point totals first until the next creature would put the total over the limit of 200. (Creatures with negative hit points count as having 0 hit points.)