"If you must know, my name is Osmund Saddler, the master of this fine... religious community."
— Saddler, introducing himself to Leon.
Osmund Saddler, also known as Lord 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. When Saddler attempted to influence Leon through the maturing parasites he infected him with earlier, much to Saddler's surprise, Leon had already manage to neutralize the Las Plagas parasite within him along with Ashley's infection of the parasites. 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.
Due to his Plaga, he is capable of superhuman strength, agility, and speed, which he had frequently demonstrated during his fight with Ada Wong as well as very briefly with Leon S. Kennedy. He is also shown to be extremely durable, being depicted as taking several rounds of submachine gun bullets and not only not flinch at all, but also expel the bullets from his hand. Owing to his Dominant Strain Plaga, he also was perfectly capable of exercising near-total control over various Plaga-infected people, at least those with subordinate-strain Plaga, having done so to Leon S. Kennedy and Ashley Graham repeatedly.
Of the various Plaga hosts, Saddler apparently is the one most affected with his Plaga infection to the extent that even in his human form, he barely qualifies as a human being. As such, he frequently does partial transformations, namely turning his right hand into a Plaga tentacle, as well as forming a tentacle near his abdomen, as demonstrated with his fight with Ada Wong and his murder of Luis Sera, respectively, with the latter also being used to subdue Ada. He also has his eye appear on his mouth, as frequently displayed during his fight with Ada Wong. Probably the biggest extent of his infection is seen during his final battle with Leon, where his entire head transforms into a large spider-like creature with eyes on the kneecaps and a long three-pronged neck showing his face with the eye on the mouth, with all that remains of his human form dangling from below like a cocoon, and even then only having his left arm remaining unchanged (his right arm and both of his legs being reduced to tentacles).