Once contact with someone is made, the host body is then trapped inside a fantasy world inside their own mind, as the plant creates them a dream of a perfect life by tapping into the pleasure centers of the person's brain. Once Mongul used a Black Mercy to trap Superman in a dream state. And he dreamed of a perfect life where Jor-El's prediction that Krypton would be destroyed proved to be a false alarm, and Kal-El grows up on Krypton and eventually marries a woman who is basically a composite of Lois Lane and Lana Lang.