The Machine World was the world of the Machine Race. It existed for millions of years and reached the peak of prosperity before the Legendary Era. At that time there were countless experts on the level comparable to God-Monarchs and Godkings and even many existences as powerful as Immortal Emperors.
Somehow, Machine World Overlord caused the Machine World to engage into a terrible war. In result the world's population was slaughtered, all cities and citadels were razed to the ground; entire world was transformed into the desert full of nothing but corpses and ruins. Even its sun was penetrated in the middle, while the moon was split in half.
The history of the Machine World was mostly forgotten in the following eras, with very few clues left in legends and myths. Even Dark Crow, after gathering information for millions of years from all around the Nine Worlds, obtained only a tiny bit of understanding and still knows very little about the Machine World.
Only Machine World Overlord survived destruction of the Machine World. With the help of the Myriad Age Tree he preserved his life for millions of years, waiting for someone to come to the Machine World.
Mortal Monarch can be said to be another survivor, but he actually left the Machine World before its destruction and don't know much about it. Later on he created the Heaven's Secret Valley and passed down the Machine World's craftsmanship to its members. Even though he remembered very little, it was enough for the Valley's mechanical prowess to be considered as the best in the Nine Worlds. In a sense, the Heaven's Secret Valley can be considered as the Machine World's successor.
Somehow two objects from the Machine World, the Mechanical World Vessel and the Insect Pipe, ended up in the Lesser Imperial Devil World and were obtained by Vedas Vajra and Devil Lord respectively.