Location the gap between the corruption and the crimson.
"Long ago" the Brain of Cthulu begins, "When the world was just created, its land was in chaos and light and dark energy was moving chaotically all round the world, so much so that living beings had a hard to surviving in such an environment As our world Terraria has kinda of a will of its own, it manage to take action and reorder the light and dark energy of the world, into the corruption, crimson and hallow, by redirecting all the light and dark energy and creating these 3 biomes, the world became much more hospitable for living beings, however as time went on, the three biomes gained their own sentient, that is what me and the Eater of Worlds is, we are the spirits of the crimson and corruption."
"Wait, hold on." Atlas interrupted and looked up at both the Brain of Cthulu and Eater of Worlds, "I kinda know the part where you two are like the spirits of the land, but what the fuck is the hallow... wait is it that rainbow land that I just came back from?"
"If you wanted to know then let me finish" The Brain replied, seemingly upset that he was interrupted. "As I was saying, we are the spirits of the crimson and corruption, and we are the land was created from the dark energy. The hallow, however, was created from the light energy. Just like us the hallow eventually gain their own sentient, however unlike us she does not understand the concept of her existence as much as we do. While we understand that we are the existence that are meant to keep the darkness away from living beings and to balance the lights of this world, the spirit of hallow, who is known now as the Empress of Light believes that light is a much more superior existence compared to the darkness and anything else and she then decided that she needs to purified the land by spreading her hallow to the entire world, which was a horrible idea due to the fact the world needs both darkness and light in order to not fall into destruction and chaos."
"Yes the Empress of light is a manic that does not understand her existence and our existence and think of us as disgusting impurity that needs to be purged, this also includes all creatures and plants that are not effected by any of us. She is trying to take over the world which would destroy it. As such we went into a war against her trying to stop her from taking over the world." Eater of Worlds continues "This event happen hundreds of thousands of years ago."
"Wait the War has lasted that long?" Atlas asked, shock not only by the age of the two existence infront of him but also at the fact that they had a war with an existence of equal age.
To answer Atlas question, The Brain of Cthulu replied with "No, the war did not last that long, infact it ended in just a few years, with the result of us losing."
"What?! if you guys lost then how are you all still here and the world is not destroyed?"
"We did lose, the hallow manage to engulf everything in the world, however it seems like the will of Terraria realised something was wrong when the world was overflowing with that much light. As inorder to seal the hallow and all these light energy, it decided to uses the remaining bit of our power that still floats around in the atmosphere inorder to create a seal that removes all the dark and light energy in the world directly into its core which is now known as hell. A guardian of the underworld was created as the seal and as long as it doesn't get defeated then the hallow will not return to the world, however we would also have to be weaken as most of our energy was used as the seal for the hallow and also if we were too powerful then the world will have too much darkness which would cause the will of Terraria to act up on us and we wouldn't that."
"So you mean that seal was broken and now the hallow is about to take over the world again?"
"Well yes but this time the result might be different, because all the previous time me and the Eater of worlds have not been able to work together as we are still different biome and existence, but now that we have you now our chance of winning the war is now a possibility." The Brain replied. "Infact this is actually what we have been raising you for this entire time, honestly I wished we had a few more years as i believe you are still abit young but I believe we can wait no longer as the hallow has reached our doorstep."
"Wait, so you two raised my up all because you two wanted me to fight a war for you?"
"Yes, are you mad at us?" The Eater of World asked slightly concerned at whether Atlas would feel as if he was nothing but a tool for them.
"No I am not" Atlas said, surprising the two ancient being." I would of probably died if you two had not pick and raised me, also I owe you all for all the things I have now so I don't care about what you were gonna use me for, to be fair it also a war to save the world, there is no way I would get mad at being force to fight, its for my own sake as well." Atlas explains.
The two being looked at Atlas, touched at how their adopted human didn't hate them for raising him for war, and decided that it they should called it a day, as tomorrow they would have alot to do, from gathering their armies to planning strategy. In Atlas case he went back to his room and gathered everything that he had into his inventory and his pocket dimension that was taught to him by the Brain of Cthulu. From Weapons to accessories he gathered everything as he did not want to accidentally forgot anything. Tomorrow would be the start of the war and he will be prepared for it.