They say that Mt. Dia has been around since the beginning of Dúlra's existence and that it's birth represents the creation of Entei in the region, as following with the known myth that a new Entei is born with the awakening of a volcano in the World.
However, only a few years back a scientific investigation team was granted the ability to study the previously unexplored active volcanic core of Mt. Dia by the now succeeded former Champion of the region. Expecting to find nothing but simple examples of fire type life and a barrage of molten rock, the results of the endeavor could have been no worse than what they were and some say that screams could be heard from the mountain that day. In one of the most well known tragedies to occur in Dúlra over the course of recent years, only one man of a research group consisting of ten members and seventeen pokémon returned from the volcano that day. He brought with him his research and the results of such were quite grim.
The man claimed, that for the first few days of the expedition near the surface and outer core areas of the volcano, things had gone swimmingly without a hitch. Interestingly enough, they had found tracks belonging to an unknown pokémon, unlisted in the Dúlran pokédex at the time. Perhaps even more interestingly, it was revealed that Mt. Dia had (very unusually for such a large volcanic structure) been formed without an eruption, starting it's life cycle in a dormant state as if it had been made the way it was by a specific design. To gain further information the group final broke through to and began to study the inner core of the volcanic mountain.
That was when "it" attacked. The man raved and lamented over it... some terrifying monstrosity of molten fire and steel, it easily defeated all of the pokémon and people who stood against it and showed no remorse as they fell into the molten rock it so easily strode through. The young man whom had survived was lucky to have escaped with his life.
Upon greater study and speculation from a distance it came to people's understanding that following Mt. Dia's eruption some 20 years beforehand, the once dormant Mt. Dia had become far more powerful and active. The molten core now being in it's raging state had drawn the attention of a very powerful pokémon whom may have been sleeping beneath the Earth for eons before choosing Mt. Dia's core as it's domain.
This pokémon was Heatran... and to this day travelers and residents alike are all but forbidden from setting foot into the core of Mt. Dia, for fear that the wrath of the fiercely territorial pokémon might cause a severe loss of life once more.