"Killer petal" is a title based upon conjecture.
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In 2016, a "Killer petal", an extraterrestrial plant petal-like being, floated through a crack in time and found its way to Earth.
Upon feasting on the blood of it's first victim the petal was able to multiply, thus creating more petals. The new petals would go on to find more victims enabling them to multiply thousands of times in the space of just one day. (TV: Co-Owner if a Lonely Heart)