Due to the Events of Legends Arceus (which plays at the same time as Pokemon Legends Wilderness) something has stirred in the Cosmos. The Lord of Strange Matter has appeared and its influence is changing the land into an environment that suits it becoming a sort of hive mind.
Steria is a Dragon Fairy type Pokemon and usually resides in its own dimension where it overlooks the creation of matter.
The protagonist of Legends Wilderness actually meets it in the first few minutes of the game when it crashlands in the region. Here it takes over the protags arm which gives his arm super human abilities (like ashitaka from mononoke).
It creates its own suitable environment in the protags village called "The Core"
by Delicious-Pop-9063