In the legendary past, before Monster Balls were made; an aura-guiding hero Pokémon named Lucario sensed two groups of armies about to clash, and a threat of a massive war in front of Oldoran Castle in Kanto that would leave no survivors. He transferred this message to his master, the legendary hero Arlon, while he was being attacked by a violent group of Hellgar. During the battle, his sense of sight was lost and he was rendered unable to see. He used the detection of his Aura, and so with the offensive Wave Bomb, he eliminated them. Though by the threat, the queen of Rota, Lady Rin was resolute to die with her civilians, and so Arlon made a choice.
This movie contains 1 potentially triggering events.
Many of the human characters are eaten by the Tree of Life, including kids like May. She ends up fine but has a tearful goodbye with her Pokemon as she gets absorbed by goo.
Master Aaron has been dead for hundreds of years. He isn't Lucario's dad, but he was very close to him in a very teacher-student way and might feel similar to some viewers.