During the invasion of Japan, Lelouch and Nunnally leave Suzaku and accidentally get into trouble (some random and not worthy of mention), Yhwach saves them with the condition that: "When you achieve all your goals, when you realize that there is nothing for you on Earth and there is no one left, then you will join me Lelouch," Lelouch agrees and Yhwach erases their memory of himself. When Suzaku kills Lelouch, his soul leaves his body, but he still doesn't understand anything and thinks that this is his punishment: to observe the world but not be able to interact with it. Immediately after this, Lelouch sees His Majesty in the sky and after a short dialogue, he joins Wandenreich. During his training it is very difficult for him since he is only half Quincy (Gemisht) and he needs to try twice as hard to keep up with them. One day, Robert Accutrone approaches him and speaks about his mother: She was his daughter and a Quincy, but she did not know about it because she decided to distance herself from him before she learned the truth about her origins. Eventually, after 3-4 centuries of training and fighting, Lelouch becomes Sternritter and Grand Master of the Wandenreich, and Haschwalth is simply an advisor to the king and a member of the Schutzstaffel. The Letter given to him by His Majesty "Chessmaster" which allows him to analyze the tactics and strategies of his opponent the longer he fights with him. So the longer Lelouch fights the more possibility for him to win.
Now a few presets for building the world:
1) The world of Bleach and Code Geass are not connected in any way, the only reason why Robert Accutrone and His Majesty were able to penetrate it is a natural seikaimon that is located somewhere in the world of Bleach.
2) Time passes very differently in the worlds, suffice it to say that those 3-4 centuries that Lelouch improved as a Quincy, in the world of Geass were only 5-7 years.
3) At first, Lelouch shared His Majesty's idea, but over time he began to doubt it and finally turned away from His Majesty when he learned all the details of His Shrift (he could have prevented all his mistakes during the rebellion and beyond, but he preferred that Lelouch would push everyone who was dear so that he has no attachments left in this world and he can devote himself entirely to the cause of His Majesty.
4) After Lelouch decided that His Majesty needed to be stopped, he could conduct his research and find out that Almighty sees everything except actions aimed at deprivation of him itself, so before receiving the Shrift (which would deprive him of Geass), he geassed himself that he only wants to deprive His Majesty of the Shrift, and not kill him, hoping that they will cope with this without him.
5) After which he began the long labor of forging an arrow, which he would then give to Ryuuken Ishida so that he could "finish" it.
A mini story that has no purpose but I want to see:
At one time, Kilge and his hunters will come to the world of Geass to return something that fell into the treasury of the Britania Empire on a tip from the Grand Master (Lelouch) with an order not to kill anyone. After which they will invade the treasury, scatter everyone like kittens and take "this" by throwing out a vague statement: "If Charles zi Britania knew what he had in his hands, the planet would have choked on blood."