You the audience hear a old and wise man talk as the DOOM Slayer walks away from the dead carcass of the Icon of Sin. "... And you will be their savior, your strength their shield, your will... their sword. You remain... unbroken... for your fight... is Eternal."
The Slayer was teleported back to his Fortress of DOOM and saw that the Demonic lava and symbols that were visible from Space are no longer on Earth. He saw a portal leading to different universe altogether. One where Heros and Heroines existed. He heads there and enters the portal.
-Earth 199999 AKA The Marvel Cinematic Universe-
-During The Battle Of New York-
The newly formed Avengers saw a blue portal open above them as all of the Chitauri and a few blocks away the Ancient One looked at the Sentinel Fortress in horror, the Chitauri roared at it.
Footsteps were heard as he walked out of the portal and all was silent before the Doomslayer rushed forward and killed the Chitauri by ripping its head off clean with his bare hands. "Lady Thor? Who is this?" Captain Rogers asked.
The Goddess of Thunder looked at the strange man in odd armor. "I... I don't know." Lady Thor said as the Doomslayer looked at the group of heros and somehow knew that they weren't a threat.
"Whatever he is, he's on our side for now." Tony said. "Lets hope Bruce doesn't fight him." They nod as the Doomslayer held onto the head of the Chitauri as he grabbed its weapon. Let's just say, the killing spree was enough to make a certain merc with a mouth cheer on.
The Doomslayer saw the legendary fem fatal herself, Natasha Romanoff, and was reminded of his old instructor, Crash. He chuckled slightly and silently thought about staying on this Earth to try and stop the Demons from Hell from invading vet another universe, he spoke for the first time of over a 2,000 years. "Captain America, Iron Man, Hawkeye, Black Widow, Thor Odinson, and the Incredible Hulk. Wow." The Doomslayer said as a Chitauri Leviathan flew overhead.
He aimed the weapon at the Leviathan and fires. The Avengers looked at the bane of Hell single handedly kill one of the Leviathans with but a few shots at it's eyes. Hulk looked at the Doom Slayer with some annoyance.
"Remind me not to get on his bad side." Clint said. Natasha and the others nodded, definitely not wanting to ever cross the Doomslayer's bad side.
The fight continued as they had a new strategy. The Doomslayer saw some of the invaders corner some civilians in a building complex and went to help. He ran inside and slammed the invader's head to the ground, breaking their skulls.
The Doomslayer heard over a comm channel from SHIELD Director Nick Fury to Stark that a nuke was headed for the city. He growled slightly at the stupidity of this 'World Council'.
He was more than ready to break them. The Doomslayer saw Tony fly to try and catch the nuke before it hit Stark Tower and started to direct it up and through the portal. Tony looked at the Chitauri Mothership explode and closed his eyes and his body fell back through the portal just as Natasha closed it on Cap's command. "Son of gun." Steve said as he saw Tony still falling.
The Doomslayer was impressed by Tony's action but knew he would not survive the fall. "He's not slowing down!" Thor said as he started to swing Mjolnir around to start flying when all of a sudden the Hulk catches Iron Man midair and lands on the ground.
They ran over to him. Thor ripped off his faceplate and Cap put his ear to Tony's Arc Reactor in his chest to see if his heart was beating, he heard nothing. Hulk roared in Tony's face suddenly as the others were mourning.
Tony suddenly woke up as and breathed heavily. "Nobody kissed me, right?" He asked. "We won." Steve said as Tony congratulated everyone on the victory and asked if they wanted Shawarma.
-Timeskip-
The Doomslayer's Fortress of DOOM was put in place above the atmosphere of Earth as the Cyborg named Dr. Samuel Hayden explained to the World Council who and what the Doomslayer is and was. The Doomslayer was felt a pair of eyes on him and saw Natasha looking at his face as he was out of his armor, per request from Hayden himself.
The Doomslayer looked at her. "What is it?" He said a bit rudely. "I was wondering why you would help us?" She asked in a way that sent shivers down the Bane of Hell's spine.
The Doomslayer's eyes roamed over her body for a moment to see if she was lying, he found no signs of lying.
"I'd rather not say." He said as he saw Lady Thor and Odin himself walk up to him. "This is him?" Asked The Allfather himself. "Yes." Lady Thor said to her father.
Odin himself looked at the Space Marine with doubts that he is the one who killed the Icon of Sin. 'What?' The Doomslayer asked him.
"So you are the one who killed Hell's Champion, you don't look like much." Odin said.
All of a sudden a odd looking portal that had sparks coming off of the edges opened up behind the two Gods. Out of it came a woman who had no hair and was wearing robes.
The Doomslayer stared at her. "Whose she?" He asked. "You may call me the Ancient One." She said. "I have seen how you fight here." The Doomslayer raised an eyebrow and looked at her with some scepticism.
Odin, Lady Thor, and Natasha looked at the current Sorcerer Supreme and wielder of the Time Stone with some confusion, mostly confusion by Odin and Lady Thor. Nat was evaluating the situation at hand.
"What do you mean by that?" The Doomslayer asked. The Ancient One looked at her apprentice as Strange showed them all of what the Doomslayer has done in Hell.
Odin and Lady Thor looked impressed by the amount of kills the Doomslayer has racked up over the centuries being in Hell. Natasha, Tony, Cap, Hawkeye, and Dr. Banner looked like they would be sick. Strange also showed them a peck into his future here. A battle against a Titan who somehow knew him and was afraid of the Doomslayer.