It is said that deep in the subways of Unova, where elite trainers gather for challenges which dwarf even the League championship in difficulty, there is a train haunted by a menacing Froslass. Some say the Froslass carries the spirit of the greatest of trainers, who set the record for successive wins and has returned from the dead to defend their title. Others claim the opposite; that it was the ghost of a trainer who never made it far, or a pokemon discarded for being too weak, and seeks to throw a wrench into the plans of the greats as a form of revenge.
Admittedly, no pokemon is invincible, but this Froslass is among the most formidable of foes. Its ice attacks are said to be so cold they bring the train itself to a halt, throwing pokemon used to that inertia headfirst into the walls; against trainers who love birds, dragons, plants, and ground-types (or worse, pokemon like Garchomp or Torterra who combine those categories) the ice itself is just as deadly. Their speed is second to few, and if damaged and faster they unleash a fearsome destiny bond which will carry their foe's soul with them away from the battle.
The trainer of these Froslass – typically a beautiful young woman, wearing the uniform of a Lass – is a phantasm these pokemon create, for wild pokemon would strictly speaking be disqualified after the subway. Many trainers wish to speak with them after the match, either to talk tactics or simply drawn by their beauty, only to watch them slowly fade away. The Froslass's teammates, however (if they must be used, for Froslass often win alone) are not illusions but extremely powerful pokemon in their own right, typically veterans of the battle subway who outlived their trainer.