From Bulbapedia, the community-driven Pokémon encyclopedia.
The Dragon Holy Land (Japanese: 竜の聖域 Dragon Holy Land) is a grassy, forested area with various flora, and rock cliffs walling it. It is a paradise in which Pokémon reside peacefully with one another and sometimes train to become stronger. It is only accessible through a waterfall at the end of the Dragon's Den. The part of the Dragon Holy Land immediate to the Dragon's Den entrance is a grassy, forested area with various flora, and rock cliffs walling it with the occasional stream.
Additionally, it is home to a very old Dragonite that belonged to the very first Gym Leader of the Blackthorn Gym. Since the Gym Leader passed away, the Dragonite became the unofficial "guardian" of the Holy Land. Clair claims that every flower in the Dragon Holy Land stays in bloom all year round, and that many would believe this to be attributable to a mysterious power from the Dragon Pokémon, and indeed, there is copious amounts of flowers.
Near a lake, there is a vault-like structure, the Dragon Shrine, that is situated between two columns. The Dragon Shrine houses a sacred golden bowl of fire. This is said to be the first fire the dragons gave to the inhabitants of Blackthorn City.
It has appeared in four episodes of the anime. Its first appearance came at the end of Fangs for Nothin', and it was the focus of Great Bowls of Fire!. It made a brief appearance in the flashback of A Pokémon of a Different Color!. It again appeared in SS028.
Pokémon seen in Dragon Holy Land
In the manga
The Dragon Holy Land in Ash & Pikachu
In the Ash & Pikachu manga
The Dragon Holy Land appeared in The Dragonite's Imperial Wrath.