When Mergo spoke, Reshiram kept his head down, eyes closed, as he deeply contemplated what he was saying. Taking in every word, maybe even running the words over his head again from different angles. It was a careful posture, one that wouldn't leave any details open, should he spend all the time in the world on it. But he didn't have that, so perhaps he was only considering as much as he could.
"This is a question that Zekrom had asked me a while ago, for a very real thing that had happened some thousand years ago." Reshiram opened his eyes. "Long ago, there were two villages that lived along the same river. One was a village that lived upstream and took pride in their power and their prosperous populations. Downstream was another village that struggled to thrive due to fewer fish to hunt and contaminated waters to wade through. Famine was settling in, and the downstream villagers were becoming desperate. They soon began to steal from nearby villages, including the one upstream.
"The upstream village investigated the cause of the matter. Their experts determined something grave, and their findings were spread not only to the upstream village, but to the downstream ones as well. There was a great beast below the upstream village, yet above the downstream village, that was tainting the waters and devouring all the fish. The upstream village was very sure that they hunted sustainably, and they certainly would not be at fault for contamination when their village itself was so clean. But the upstream village did not have the resources to hunt this beast alone, and so they called upon the assistance of the downstream village to help.
"They traveled into a strange cavern that radiated an ominous aura the moment they entered. Deep inside was a gargantuan beast, indescribable to any kind of Pokemon, that gravely injured several of the upstream Pokemon, and injured some of the downstream fighters as well. But they were heroic, and fended the beast away with attacks from a distance after seeing that physical contact made the fighters lose fur and perhaps even limbs, had they not been careful.
"The beast fell. And over the course of a few moons, the river was clean and bountiful, even for those downstream, and all was peaceful."
Reshiram looked at Mergo, finally opening his eyes. "What do you believe truly happened?"