TL;DR

Originally, humans did not exist in the world, and so legends existed as just pokemon – many of which had an animal level of intelligence. Everything was stable until humans were introduced by the Sky Mother, and said humans became advanced enough to team up with pokemon and build societies.

Humans were engaging with legends in a mostly negative way, and some legends had even been killed. To create balance, Arceus created a ‘law’ that made legends both human and pokemon, and the law was slowly set forth in the world.

Existing, living legends were granted with human-level intelligence, as well as the ability to shift into a human form, while the legends who had been killed had their powers and role appointed to an emissary – a revived human who had a connection to the legend, somehow.

In the way it currently works:

Legends can shift between human and pokemon forms, as well as a god form that is a bit of a mix of the two.
Legends only die if both their pokemon and human forms are taken out, or die immediately of their god form is defeated.
If a legend is killed, a new emissary is chosen randomly from a dead human with a connection to the legend somehow. This can be as arbitrary as ‘they went to a church for them like once’. It Just Works!
- Dialga and Palkia don’t follow this rule, and instead, a random child will be given the role from birth and awaken their power sometime later. This is an even WORSE decision but Arceus trusts a New Person with the roles of Dialga and Palkia more and an Old Dead Person.
All legends human forms stop physically aging at age 21 and cannot die from old age. If the revived emissary died before then, they’ll continue to age up until that point.
- Legends are able to freely modify their human form, so they can appear younger or older if they so choose.
Legend’s dual identities are largely unknown and kept secret from the majority of the population, who only really knows them as pokemon. The only people who really know are either friends of the legend themselves or certain religious groups. It’s kept a secret because those who let others know too openly tend to be hunted down.
Only one of a certain legend can exist at a time, with some exceptions.
Mythical pokemon are also considered legends, save for any that fall under exceptions.

Active legends

Exceptions

Origins and universal powers

In the beginning, it was only pokemon.

Arceus created the original ancestor mew, along with the other legends to keep order over their various domains. Humans did not exist, and many pokemon were at an animal level intelligence, including many of the legendaries. This never caused many problems, because pokemon were the only creatures who existed.

But when Arceus met with the Sky Mother, she gave their world humans as her gift. At the time, the humans were relatively disorganized and on about the same level as the pokemon advancement-wise. But when the humans banded together with themselves as well as pokemon, they were able to create large communities where they could stay instead of constantly moving. Humans had the skill to coordinate teams, so pokemon would join and begin living with humans as companions and helpers; the two species had knowledge to mutually improve.

Because of this, an issue soon arose: while most pokemon were willing to be friendly with humans, many legends were not – and to have an all-powerful divine being on bad terms with a population is, well… not good. Legends and humans, along with teams of pokemon, would often engage with eachother, and quite often for the worse. In addition, some groups of humans had actually found out how to kill legendaries.

At the point in which this happened, Arceus has already severed their emotions, but they knew balance couldn’t be kept in the state things were, and decided to unify legends as beings of both human and pokemon.

Universal powers are shared by all legendaries, regardless of what other powers they may have or their overall strength level.

Dual forms:

Legends can shift between a human and pokemon form, and also possess a god form that is a mix of the two and only used for serious business.
- Pokemon forms can differ from emissary to emissary of the same legend – one might be discolored, smaller, etc.
- Emissaries cannot use any kind of moves or powers in human form.
- The shift is very fast if not instant, more akin to magical girl transformations (including the glitter) – though experience comes in to play when accounting for energy loss or disorientation afterwards.
If they choose to, legends can have a ‘disguise’ pokemon form, as to not reveal themselves instantly.

Conditional immortality:

Legends cannot die from old age and stop physically aging at age 21 (pokemon forms are generally ‘ageless’ and do not change appearance over time).
Legends can be killed if:
- Both their human and pokemon forms are killed.
  - If one form is taken out, they can regenerate it in time – this can take anywhere from a couple of hours to a decade, depending on experience.
- Their god form is killed.
- In the rare case where a legend is caught, the human soul is severed from the pokemon and the emissary becomes a mortal human.

Traversal between planes:

All legends are free to travel to most of the planes as they please, excluding the hall of origin and distortion world.
- Major legends are allowed access to the hall of origin, and are expected to give Arceus updates on their region.
  - Major legends include Ho-Oh & Lugia, Rayquaza, Kyurem, Zygarde, and Calyrex.
Legends cannot shift into human or god form while in the pokemon world.