Sin duo

The Sin duo, also known as the Legendary angels, are a collection of four Pokémon that embody good and evil. Due to thier past, they are the only two legendary Pokémon known to evolve and are unique to the Hora region. The duo consists of Sinne and Luumin, who evolve into Sacrelivo and Lagralia respectively. All four Pokémon are uncatchable Pokémon.

Myth
"''During the birth of the Pokemon universe through the will of Arceus, Lagralia and Sacrelivo were born as the embodiments of good and evil. These two pokemon proved to be too powerful to exist without consequences, thus, Arceus used the last of its power to change the two into Luumin and Sinne, weaker versions of their true selves. It is said that Sacrelivo is the one who created the Dark-type Pokémon. It is the only truly "evil" Pokémon, and will go out of its way to bring harm to others. It is said that Lagralia is the one who created the Normal-type Pokémon. It is the only truly "good" pokemon, and will go out of its way to help those in need. Though their powers are suppressed, their personalities are not, in their pre-evolved forms. Luumin appears around those who are lost and guides them in the right direction. When Luumin is brought to the Sacred Sanctum in the Hora region, it returns to its original, true form. On the other hand, Sinne appears around those who are lost and proceedes to leads them farther from the way out, only to leave them in total helplessness later on. When Sinne is brought to the Condemned Chamber, it returns to its original, true form. Legend has it, if these two Pokémon ever return to their true forms, Arceus will appear to bring judgement upon the one that summons them.''"

Trivia

 * All credit for these four Pokémon goes to a user named "BlueGhosty" on DeviantArt, as is the images associated with them.
 * Ray's parents are said to have come up with this myth after discovering something incredible in their work. Unfortunately, the day after they reveal their myth, they've been kidnapped by Team Rocket. Their whereabouts are currently unknown.
 * The Sin Duo are the only legendary Pokémon created by Arceus without a third member to their group. Due to lacking a Trio, they lack a Trio Master, though Arceus can be thought of as their master due to having created them. Yet at the same time, Taijuti is the only Legendary Pokémon able to quell them. This would make them the first Legendary Duo with a "Duo Master(s)".