|The title of this article is conjectural.|
Although this article is based on canonical information, the actual name of this subject is pure conjecture.
"Calamity World" is the original world of Ehit and his friends had originally come from before migrating to Tortus. Its true original name is unknown and its current name was given by Hajime due to his disdain for Ehit. The World Tree Branch of this world is named "Grudge Tree" due to the grudge that Hajime Nagumo assumes that tree would have for the people that destroyed its world.
The sentient inhabitants of this were similar to the humans of Tortus and Earth. And at some point, their civilization advanced so far in magic and technology that they ended up destroying their planet due to having presumably beginning to abuse their world with their advance capabilities. This then eventually lead to a group of survivors of the world ending calamity that destroyed that world escape to Tortus. And from there they began to use their knowledge, advanced technology and powerful magics to advance the various civilizations of Tortus to a much better standard, leading to many of them to be regarded as Gods by Tortus's native races.