While Pokemon Legends: Arceus isn't open-world, it is more open-ended than most other Pokemon games. Players are generally free to tackle the game's objectives, outside the story, whenever they want to. The Pokedex is something that players can tackle at any time, with some exceptions. Players will need to progress to certain ranks on the Pokedex to be allowed access to the next area. However, one Pokemon Legends: Arceus player took that open-ended objective to the extreme in the first area of the game.
The Pokedex in Pokemon Legends: Arceus behaves differently than Pokedexes in other Pokemon games. In previous games, simply catching a Pokemon or evolving it was enough to fill out its Pokedex entry. In Pokemon Legends: Arceus, there are various tasks players need to accomplish in order to fill in the Pokemon's respective entry. These can range from catching more of that Pokemon, seeing them use certain moves, feeding them food, etc. These tasks will help you rank up by earning you points towards Pokedex completion.
There are 10 ranks in Pokemon Legends: Arceus, and the player is expected to hit level 2 before leaving the first area, the Obsidian Fieldlands. However, Reddit user Kyarmak shared a screenshot that showed that they have hit Star Rank 9 without leaving the Obsidian Fieldlands. To accomplish this, it is likely they would have had to capture every single variety of Pokemon there, including the ones from the space-time distortions, raised them, evolved them, and completed entries from the evolved forms' entries too.
There are a number of rare Pokemon like Munchlax and Happiny that can appear in the Obsidian Fieldlands, but their appearance rates are low, which requires patience on the player to reload the spawn by fast traveling or returning to Jubilife Village. To reach Star Rank 9 in the first area requires a lot of time and dedication on the part of Kyarmak.
Like most other games with open-ended objectives, it's pretty common to see players try to test the limits of what they can do given certain restrictions. With games like Breath of the Wild, players are still learning new things they can do to progress in the game. It is unknown if Pokemon Legends: Arceus will have that kind of longevity, but it could still have more secrets for players to uncover.
Perhaps some might consider it a waste of this player's time to hang around the starting area with this kind of goal in mind, especially considering they must have bought Pokemon Legends: Arceus early given how long it takes to do some of the research tasks. However, the great thing about games like Pokemon Legends: Arceus is the freedom it gives to players to play however they see fit.
Pokemon Legends: Arceus is available now exclusively for the Nintendo Switch.