Dr. Isabella Horn is the main antagonist in the 2019 horror film Eli. She appears to be an immunologist who specializes in treating children with failing immune systems, and runs a "clean house" for this purpose. She is actually the leader of a group of nuns, who are attempting to "cure" children fathered by Satan.
She was played by Lili Taylor.
Dr. Horn has been an immunologist for 25 years, and has been running a treatment center for children like Eli for the past three years. She deceives her patients about what's wrong with them, telling them that their immune system is failing and that she will use gene therapy to make them healthy. She subjects each child to torturous medical experiments, where she injects them with holy water and tannis root, which leaves the child in agonizing pain. When the initial treatments fail to work, she moves on to Treatment 3, a ritual exorcism which kills the patient. She then disposes of their bodies in an underground burial chamber, with it being revealed that at least three were murdered this way.
She has been working with Eli's parents for some time, and has instructed them to lie to him that he has a failing immune system and must be isolated from the world. She has also charged Eli's parents a sizable amount of money in order to treat their son. When she meets Eli, she seems warm and friendly to him, but won't answer his question about how many patients she's actually cured. She tells Eli that they will inject a modified virus (actually holy water) into his immune system to "reset" it. As Eli stays at the treatment center, the ghosts of her other patients (and Eli's half-siblings) try to warn Eli about her intentions, and Haley, his half-sister, also tries to warn him about what's going on. When Eli tries to tell his parents about this, Dr. Horn claims he is hallucinating due to the medication, and lies that none of her other patients had seen ghosts.
The second treatment leaves Eli in intense pain, and Dr. Horn tells his parents that he had a bad reaction to it. Eli continues to see the ghosts, and nearly escapes from the facility. When Dr. Horn catches him, Eli accuses her of lying, and tells her that he knows about Perry (one of her previous patients). This alarms Dr. Horn, who decides Eli knows too much and has to be silenced. Despite having earlier claimed that the gene therapy was working, she now tells Eli's father that Eli is not accepting treatment. They decide that the third, fatal treatment is necessary. She deceives Eli's mother about the nature of the third treatment, telling her that Eli's body is fighting the gene therapy, and that his "delusions" of seeing ghosts prove this.
Eli becomes suspicious of Dr. Horn and investigates further, discovering medical files detailing the deaths of her previous patients. He tries to warn his parents about this, but Dr. Horn has convinced them to go through with the final treatment and has Eli sedated, though it doesn't work on him. Eli is taken to the operating room for the treatment that will end his life. However, his mother discovers a secret chamber where Dr. Horn has disposed of the bodies of her previous victims. She tries to stop Dr. Horn, who claims that Eli is "too far gone" but that she can save his soul. Eli's mother asks her husband to help their son, but he betrays her and gives Dr. Horn a ceremonial dagger to kill Eli. Dr. Horn throws holy water on Eli, ignoring his pleas that he's done nothing wrong, and attempts to stab him in the heart while his mother is forced to watch. However, Eli's powers manifest, and he telekinetically throws the dagger into Dr. Horn instead, before setting her and her assistants on fire.
On the surface, Dr. Horn seems to be warm and caring toward her patients, as she tells Eli not to be scared and that she will make him better. Even when the treatments leave him in agonizing pain, she tells him that it's supposed to happen and that everything will be okay. However, she is actually a very cold person, as she is willing to torture and murder the children in her care, believing that she is doing God's work. It's implied that she is motivated less by her faith and more by fear of anything she doesn't understand, as Eli and the other children showed no desire to hurt anyone, but she views them as evil for existing. When Eli tearfully asks what he did to deserve death, Dr. Horn tells him he has done nothing wrong, but she believes he will.
She is willing to manipulate and lie to Eli and his parents, and is also said to have charged them exorbitant amounts of money in order to see Eli, suggesting that she is also greedy. She also shows herself to be a hypocrite, as she condemns the Devil as the "master of lies" despite repeatedly lying to others herself.