Basculin In Hisui Evolves into Basculegion. This Evolution Occurs when a basculin is possessed by the souls of other basculin from its school that could not make it upstream.
Since The Hisui region is presumably takes place in Real world equivalent of hakaiduo In Japan, from 1860 to 1890.
Basculegion, If we seperate it's name we get Bas and Legion.
The bass part is obviously inspire by the bass fish, and legion refers to the souls of other basculins inside Basculegion. There are more inspirations for this meta cool new Pokemon, like salmon, the streamlined body shape of Basculegion points to a similarity with salmon, since haikadou has one of the biggest salmon fishery in the world, so of course a pokemon with some inspirations towards salmon makes so much sense to me. The Ainu people valued salmon with so much importance, since they were essential to the survival of the Ainu people. They even have a Kamuy for salmon. Also Kamuy is the belief or entity by the Ainu people, back to Basculegion though, the Ainu people have a Kamuy for a Divine Fish that they offer sake to, it suits perfectly since the Ainu people have the belief of souls as well. Basculegion most definitely took inspiration from it. In addition their is a species of salmon in hakaidou that are HUGE and are currently Endangered, which makes perfect sense that they only exist in the past sinnoh region.
Basculegion is one of the cooler new Pokemon shown on the trailer for Legends Arceus and I LOVE IT, It's design, It's Lore, It's Origin and It's Pokedex entry just SCREAMS cool. I feel that Basculegions lore goes the darker route from the gyrados and Magikarp lore, and it's too good. The water/ghost typing is extremely cool and underrated since the only other Pokemon under this type is The whole
frillish line, which I like ,but those 2 pokemon is kinda..... but Basculegion is a Perfect edition to the type, it's probably extinct in current sinnoh which is kinda sad because of the effects brought by us humans even in the pokemon world