"If you must know, my name is Osmund Saddler, the master of this fine... religious community."
— Saddler, introducing himself to Leon.
Osmund Saddler was the leader of Los Iluminados, a now-extinct cult that formerly operated in rural Spain. Under his leadership, it sought world domination through the use of a mind-controlling parasite called "Las Plagas".
Some time before 2004, Saddler had converted the local population into his cult, whose original aim was to free the Plaga parasites from the caverns underneath a nearby castle. His plan for Ashley Graham was to infect her with one of the parasites and send her back to the United States. She was to then infect her father, President Graham, who would then control the nation under Saddler's command. He also intended to use the ransom money meant to ensure Ashley's safe return to fund Los Illuminados' propagation.
Saddler encountered Leon S. Kennedy and Ashley frequently, first setting his minions on them, then using his growing power over their Plaga-infected bodies to control their actions. He killed Luis Sera (a researcher who betrayed him) by impaling him with his tentacle tail when he was about to give Leon the pills to suppress the growth of the Plaga in his body along with a sample of a Plaga.
Saddler hired Jack Krauser (a former ally of Leon) to dispose of him after realizing that Ramon Salazar would not succeed in doing so, but it was later revealed that he didn't fully trust Krauser and was looking for a way to get rid of him. He briefly fought Ada Wong after she helped Leon free Ashley for the final time. Though he was defeated, he still managed to capture her.
"The American prevailing is a cliché that only happens in your Hollywood movies! Oh, Mr. Kennedy, you entertain me! To show my appreciation, I will help you awaken from your world of clichés!"
Finally, Leon confronted Saddler on a platform high above the island, saving Ada from death at the cult leader's hands. With no further options left, Saddler transformed himself into a massive, multi-legged monstrosity, with the remains of his human body dangling from the center of the beast. Despite Saddler's considerable strength in this form, Leon managed to kill him by using a rocket launcher that he received from Ada Wong, ending the Los Iluminados' plans once and for all.