So I saw 4 second video of Cell from Dragon Ball Z appears above over British Isles while doing his Kamehameha which bring an idea but with a twist.

DISCLAIMER: Dragon Ball Z and Code Geass are belongs to respective owners.
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After absorbing Android 17 and 18, the genetically insect-like humanoid creature/android known as Cell achieve his Perfect Form and decide to test his skills and new powers. He ironically save Earth from a large meteor with his powerful Kamehameha wave that turn it into many pieces creating meteor showers.

So he use an Instant Transmission copied by Goku and teleport into accidentally to another universe. Taking interest, Cell travel around the world and discovered this Earth has no Saiyans nor Z-Fighters but instead, he travel to an alternate Earth where the dominant Holy Britannian Empire ruled half of the world with their strong, iron fist and tyrannical rule as they abused the low class so-called "Numbers" like slaves while the rich and nobles lived luxurious and powerful. It was due of the empire's ruler Emperor Charles zi Britannia and his ideology of "strong rules over the weak" policy. Heck, he even brags about he is more powerful than God himself.

Cell frown about this. It seems there are humans worse than the humans on that timeline where Goku & other Z-Fighters lived. He disappointed that there no REAL strong fighters as he saw those Britannians were nothing more but a bunch of idiots thinking they were higher themselves than others who were weak due of race and nationalities.

Cell maybe a villain but he has morals. Even though he attacked and killed some humans while he was Imperfect form, to him they were just wasting his time for their arrogance and think they can take him down.

Without further ado, Cell teleport using Instant Transmission and appears in space over the North American continent that is now Holy Britannian Empire.

Cell: This empire was nothing but a hubris of time. Time to destroy the Britannia and show this world my ultimate power. *Does a Kamehameha pose* Kame..Hame...Ha!!!

Cell fires a large and powerful Kamehameha wave onto the continent.

Meanwhile in Holy Britannian Empire, Emperor Charles zi Britannia was standing on the balcony in the palace when he notice a large beam of light falling into skies.

Emperor Charles: Was that...God?

In the far wide ocean view, the beam of light crashes with a big explosion so powerful that made the atomic bombings of Hiroshima and Nagasaki look like a firecracker as the entire continent was destroyed, killing the emperor along with Knights of Round Table, the army, naval and air fleet and the people living there.

Cell watched this and smirks.

Cell: There. That will teach them that there's someone more powerful than they are.

After that, Cell teleports back to the world where Z-Fighters are and prepare for his own games which is Cell Games by the way.

Back on that universe, news spread around the world about the entire North American continent was engulf of flames and destruction. At Area 11 in Ashford Academy, Lelouch vi Britannia/Lelouch Lamperouge aka Zero and C.C. look shock and disbelief inside his room.

Lelouch: What just happened? How did this happened?
C.C.: Hmmm...it seems God or whatever something had enough of Britannia's rule.

Others like Suzaku Kururugi, Kallen Kozuki alias Kallen Stadfelt, Shirley, Rivalz and other Ashford students also had the same reactions like Lelouch. Even Milly Ashford and her grandfather and dean Reuben Ashford were also shock & disbelief.

In the Black Knights hideout, Kaname Ohgi and others were also had the same reactions as well as the resisting Japanese Liberation Front when they saw the news about Britannia's destruction.

At the Britannia viceroy palace in Area 11, Princess Euphemia li Britannia along with Princess Cornelia li Britannia, her loyalist followers like Darlton, Lloyd and Cecil were all shock & terrified upon the news. Prince Schneizel li Britannia who was Britannian Prime Minister along with Guinevere and other relatives were visiting in Area 11 to meet the half sisters when this news happened.

Euphemia: How can this be?
Cornelia: I don't believe it.
Guinevere: The entire continent...
Schneizel:...is gone.
Darlton: What the heck just happened?! There's no way other country like EU could launch a powerful weapon into our empire right?
Lloyd: Well...*clears throat* ..according the reports we received from the Intel, a large and powerful beam of light appears falling from the sky and then, hits the entire continent of North America.
Guinevere: What is that means?
Cecil: In other words, that beam came out from the outer space.

They looked dumbfounded and confuse. How the heck a beam of light from outer space destroys the entire continent and its empire in one swoop? Until an officer enters the room who hysterical in shock.

Officer: Your highness! Prime Minister! We have a problem!
Darlton: Let me guess. Zero appears and attack huh?!
Officer: No, sir! A military from EU staged a coup as they deposed the EU Council and install the military junta! We received today that EU is launching an invasion to Euro-Britannia!
All: WHAT?!

And so with the destruction of Holy Britannia and its main military, the remaining Britannian military posts are in disarray and confuse as many people raise their arms and launch an all out revolution. This chains of events saw this an opportunity as Zero and his Black Knights and the JLF were launch their all out attacks separately within Area 11 with the Elevens who are no longer enslaved by Britannians, also took up arms to fight for the nation. Even the Chinese Federation are not safe from the flames of revolution as the people toppled the Eunuchs and free Empress Tianzi.

Outside of Area 11, Europe was now covered in flames of revolutions after a military coup within EU deposed the council and install the military junta & they launched the invasion of Euro-Britannia to reclaim the lost states as well as the Middle East, leading up to their world's Second World War.

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And that's how Cell, one of the best Dragon Ball Z villains became the hero of Code Geass universe. 😆😂🤣