Diane De Poitier Edit
In Banished Catherine learns that Diane killed her twin daughters, Henrietta and Emone, by opening the windows in the nursery and exposing the babies to the cold air. Diane admitts that she did this out of jealousy over not having Henry all to herself (he was still sleeping with his wife despite having an affair with her), and her anger reached a boiling point when Catherine became pregnant with the twins. Catherine sneaks into Diane's chambers and hits her on the head with a shovel. Diane falls on the floor in surprise and starts to bleed. Catherine admits she was slow to put it together much to the misfortune of her poor daughter Claude. Diane decides to provoke the enraged Queen by calling her a hypocrite and asking her how many mothers’ children has she killed. Catherine then steps foward and hits her on the head three times saying that's for taking her husband from her, for tearing her family apart, and for turning her daughter against her. She grabs Diane's necklace and twistes it around Diane's neck. Diane tries to grab Catherine's face but Catherine shoves her off and kills her.
In Liege Lord, Mary discovers that Catherine and her mother Marie de Guise put a secret clause in her marriage contract, so she sets out to get Catherine back and with Francis' help turn Scotland against her mother. Later that night, she sends them to a 'whore house'. On Catherines orders guards kill the Scotsman and burn the house down in order to stop the news of the marriage contract from spreading.
In Mercy and Getaway, Catherine begins to slowly poison her daughter Claude due to the fact that she has been having hallucinations about her two dead twin daughters Henrietta and Emone. They keep telling her that she must get revenge on Claude because she sufficatated them when they were babies. Fearing the twins will kill Claude in horrible ways, Catherine decides to kill Cluade before the twins do. Kenna later figures out what Catherine is doing and begins to bring Claude food and 'accidentally' knocks the food Catherine brings over. When Claude questions Kenna, Kenna tells Claude Catherine is poisoning her. Claude says that Catherine would never do that to her. but after Kenna explains how Claude always feels bad after reciving food from Catherine, Claude goes into Catherine's Chambers and searches for the poison. Catherine admits that she has indeed been poisoning Claude. Claude breaks down crying after realizing her mother thinks that she killed her sisters. Catherine throws her arms around Claude and says that she was young and that it was okay. She looks at her hallucination of the twins and telling them that Claude was so young and that it was an accident.
King Henry Edit
In Long Live The King, Catherine and Mary work together to poison Henry after this madness has reached a breaking point. Learning that the kings food goes through 7 different tasters, they decide to poison Henry during his private mass. However, they stop the priest right before mass believing that Henry is getting better.
Mary Queen Of Scots Edit
In Pilot, Catherine fearing the vision that Nostradamus had about Francis's death, blackmails Colin into letting Mary drink a potion, concocted by Nostradamus, to let her "sleep like the dead" while he raped her, taking her virginity and making her unworthy of marrying Francis. However, the castle ghost told Mary not to drink the wine given to her at the ball. When Colin come into rape her during the night, Mary is very much awake having not drunken the wine.
In Inquisition, Catherine is offered poison instead of execution. She puts the poison into her face cream. Later, when she realizes that after her death Mary will still cause Francis to die, she breaks out of the North tower and enters Mary's bathroom and dumps the face cream in Mary's bathtub while she is bathing. Telling her not to cry out, Catherine hold a life to her throat. Mary begins to sink under the water due to the poison. Clarissa pulls Mary out of the bath before she dies.
Duke of Guise Edit
In the End of Mourning, the Duke gets into a confrontation with Narcisse over Catherine, and is accused of having poisoned Catherine and Henry indirectly through a poisoned bible. When Mary finds this out she says he must be stopped. Catherine smiles and tells her that no one hurts her family and gets away with it. Later, the Duke is riding in a carriage when it aurptly stops. The Duke gets out and discovers that there is no one in the drivers seat. Suddenly an arrow hits him in the neck. After, another 3 arrows hit him in the leg, the shoulder, and in the back.