bred with Jessica's mother. The Barron is gay pedophile. I'm not sure how well that comes through in the movies, but in the books he has an insatiable fetish for young boys. So when Mohiam is given the assignment to breed with him, she knows she will never be able to seduce him and get him to voluntarily sleep with him. So she comes up with a plan to collect a bunch of dirt on the Baron and gets proof he's stealing spice on Arrakis from under the nose of the Emperor. Then she uses this to blackmail the Baron into sleeping with her.

Mohiam goes back the Wallach IX and has the child, but the child has a bunch of birth defects and eventually dies. Mohiam is sent back to breed with Vladimir Harkonnen a second time. She blackmails him again, and he flies off into an uncontrollable rage. He sleeps with her, but it's brutal. He beats her to within an inch of her life while he's doing it. Reverend Mothers have perfect control over their bodies (it's how they convert the poisonous blue raw spice serum to something that is benign - they use their bodies to change the chemical composition of the stuff), and Mohiam uses her body to produce a virus that she infects Baron Harkonnen with while he's beating/raping her. Up until this point, Baron Harkonnen had a perfect muscular body. The virus Mohiam transmits to him makes him uncontrollably fat to the point where he can't even support his own weight without using a special belt with suspensors on it. That's why he's always in a pool of water or floating using the suspensor belt.