To fully understand the might of the Doom Slayer I have to take you back to where it all began. Where his legend was born. The beginning of most of the original legends.

The First Great Ninja War

There are a lot of misconceptions about the battles. Firstly, that the second Mizukage and Tsuchikage killed each other. This was not the case. Few people know the truth. Let us start there shall we?

The second Mizukage (Gengetsu) and the second Tsuchikage (Mu) were heated in battle. The battle had just started, both still raring to go and full of chakra.

The two stood in the middle of a clearing, sizing each other up and trading blows. The combatants had a certain hatred for one another. They were battling in the middle of a light mist, the visibility wasn't great but it was not that much of a disadvantage.

Amidst the battle a strange swirling portal opened up on the ground. A dark shadowy creature floated to the top. This strange occurrence caused both Kages to pause the fight.

The being was humanoid but their visibility, once again, was not perfect. Suddenly a pair of glowing red eyes appeared on the figure. It stepped of the portal (Which vanished) and into better light.

There stood a man? A shinobi by the looks of it. He was wearing some sort of classic shinobi battle armor. It was not out of the ordinary since Flak jackets would not become mainstream until the 'next' great war. He appeared to be staring straight down at his feet.

This armor was a dull green on the metallic parts and brown on the leather portions. He was wearing some strange form of shoe. Boots? They were unfamiliar to this time period and certainly to the two Kages before him.

This battle armor was peculiar indeed. His biceps were exposed and he was strong. any fool with a pair of eyes could see that, his muscles bulged out of the edges of the fabric. Down toward his wrist and his hands, there were smaller metal components that created a sort of gauntlet. Counterproductive for creating hand signs. (or so it appeared) it's use was just not apparent quite yet. There were pouches across the guys belt, he no doubt carried utility in there. His head was covered up, by a facial mask and hood. On the hood above what would be the warriors left eye, there was a strange symbol, it was marked in red. (the mark of the Doom Slayer, the last mark you will see) There was some sort of visor or goggles covering the warrior's eyes. It was a dark color, almost dark blue.

Lastly and certainly not least. There was a massive curved blade fixed to this shinobi's back. It was surely a great sword, the likes of which had never been seen before.

"who are you?" Gengetsu spoke. Both Kages visibly upset at this interruption.

"Gengetsu... There is something weird about this one"

"What do you mean Mu?"

"I've never seen chakra like this. It doesn't match any I've seen, none of the great nations or otherwise... It's... It's frightening, the intensity and color. It's as if I'm staring directly into the underworld itself."

"I will ask once again Shinobi. What is your purpose here? Are you some sort of Assassin?"

This figures head jerked up. Glowing red eyes becoming now visible through this heavy mist.

Gengetsu shook his head slowly and turned to Mu. "Shall we put out differences aside just long enough to kill this intruder?"

"I believe we sh-"

A massive red blade burst through Gengetsu's chest. When? This unnamed shinobi was fast. Neither Kage even noticed handsigns.

Gengetsu faded into water, water clone, as mu jumped backwards to help distance himself from this unknown foe. They got an even better look at him and his "weapon".

The massive sword on his back had not even been used, this 'blade' was some weird chakra. It extended off of the warriors' forearm. It slowly dissipated and this unnamed man struck a fighting pose. Challenging the other two leaders.

Mu and Gengetsu looked at each other and then back at the masked assailant.

The battle was on.

Gengetsu lay dead on the ground, and mu on the verge of death. He had split in two to avoid a fatal blow but the opportunity the join had not emerged... until now.

This warrior was unreal. Hand signs were virtually nonexistent. If he was making signs. They were too fast to be picked up, even by mu's advanced eyes. He was a skilled Kenjutsu user. His ability to control weapons was once again unreal. The attacks the warrior took didn't appear to do any damage. There was no indication at all except soft barely audible grunts from within the warriors mask.

Mu had come to this. He had only one option left. Dust Release. If the jutsu worked (or failed at this point) It would kill Mu, and even on the off chance it didn't take his life this warrior would end him. He would have to be quick, there was no time to do the full preparation. He would have to start the jutsu and instead of shooting it. He would have to just touch the warrior with it. This shouldn't be a problem however, this guy loved up close and personal combat.

Mu formed back together as one. He was breathing heavily and covered in wounds. He started the jutsu as this warrior marched on him. Two small white spheres forming in his hands. Mu's breathing got worse as the jutsu continued. The assailant was closing in on him. He was strange in his movements. Very leisurely, even while fighting, unless absolutely necessary to dodge an attack.

He no doubt saw what Mu was doing. He did this with every attack. Just waiting for it to hit, then allowing it to either hit and shake it off or dodge it at the last second. This person. No, this thing was not human. No human reacted like that. He was a robot on a mission to annihilate these two Kages. His purpose and reasoning behind it was still unknown.

The Warrior arrived at Mu. He grabbed ahold of both his wrists and pulled them apart to attempt to stop this jutsu. However, the jutsu did not break and just widened. It was pushed slowly forward and began clinging to this person's arms. It ripped at the guantlets, tarnishing them and tearing the fabric.

The dust release orb continued to go for a few more inches before finally a large red chakra emitted from the assailant's chest and consumed it. He had just survived the Dust Release jutsu. Any other shinobi would have been reduced to the molecular level immediately but not this guy. He was UNREAL.

With a quick movement the warrior grabbed both of mu's wrists and held them in one of his hands. Mu fell to his knees. He knew it was over, He just wanted to know one final thing before he died.

"Who are you?" Mu spoke as blood dripped from his bandaged chin. Most of the bandages in that area already had been soaked in it.

The unnamed assailant paused and reared his other arm back to prepare to lay the final blow into this Kage, this Trouble maker. His fist tightened, the metal 'gauntlets' around his fist now more evident in their use. They were used for ripping and tearing.

"Doom"

With a single word the final blow hit Mu.

When the two Kages were found there was a strange symbol that had been written in blood. It was the mark of the doom slayer and would appear where ever he worked, and the scenes which the symbol was found were always brutal. In order to keep the 'mystery' hidden and the people satisfied and unworried, they were told that both Kages and killed each other in battle.

The next thing the Slayer did was Assassinate the second Kazekage. This strange symbol was once again found on the wall next to the body. Not much else could be said about that. Like a thief in the night the Kazekage was murdered and not a trace of evidence of who had done it (other than the strange symbol) was found.

Lastly, the Gold and SIlver brothers would have to pay the ultimate price for their crimes. Their death, along with the symbol was also a mystery to those who found the bodies. However, it was not that big of a concern, those two had deserved what came to them. The symbol and the mysterious deaths went down in history and no more symbol would appear... that is until the next great ninja war.