Kisenian was a flower that would take control of a person (like Fiore) and turn their "pure heart" into a dark one. The person would wander about the Earth, carrying out Kisenian's plans. After everything had been absorbed/destroyed, the planet would explode.

Kisenian was left on the comet (probably remainders of planet that disappeared) where Fiore found her. Fiore then got the idea (once he picked the flower) that everybody had left Mamoru all alone. That's where Fiore's heart became dark.

Eventually, Sailor Moon showed Fiore the truth about Usagi and Mamoru's friendship, killing Kisenian.