Gen 10's story is pretty dark (at least by Pokemon standards), with death and the afterlife being a central theme.
The game has a more classic evil team called Team Veil, led by a tragic villain who wants to use the box legendary to resurrect their child who was implied to have been killed by the actions of a previous game's antagonist. Unfortunately, doing so would cause the living world to merge with the afterlife and create mass chaos, so the player needs to put a stop to it.
During the game's climax, the player and the rivals are confronting the leader of Team Veil, who sets off a device of some sort that kills them all (and all of their Pokemon). The player and the rivals wake up in the afterlife where a few notable dead characters help them find Team Veil's leader. The player battles them and after their defeat their child helps them realize the error of their ways. The box legendary shows up to resurrect the player, rivals, and Team Veil's leader, but the leader chooses to stay to be with their child. The player and rivals (and Pokemon of course) are sent back to the living world and then the player takes on the league. The box legendary can be caught in the postgame.