Battle Tree

The Battle Tree is a new feature introduced in Generation VII.

Description
From Pokémon Sun and Moon official site:

Somewhere in the Alola region, you’ll find the Battle Tree: a place where those Trainers who have completed their island challenge—or who are strong enough to have done so—battle one another. All of the Trainers you can encounter at the Battle Tree are strong. It seems you might even encounter two battle prodigies from the Kanto region...

Trainers

 * Cynthia


 * Wally


 * Red


 * Blue

Trivia

 * It seems to be a new variant of the Battle Maison.