(Requested by KeiraRennn, I just added some stuff :3)

What if before Rainbow Quest and all that happened in it, Sabre was in the Assassin's Creed universe first?

Sabre was already a prominent and skilled assassin during his first life as one (aka the very first story arc that ended in him being surrounded by guards while poisoned after killing the king). It almost came naturally to him. After he died in the castle, instead of in some kind of afterlife, he found himself standing in a bright abyss with that world's deity in front of him.

The deity had watched his progression as an assassin, and was very impressed. He had never seen someone go to such lengths an achieve so much as one, nevertheless be so successful. Acknowledging his skill and in need of some help, the deity had a deal for Sabre.

The deity knew that dangerous and cruel-hearted people were hunting for its powerful artifact, the Apple of Eden (if you know you know >v>). Not wanting any mortal to have access to power so threatening to all of the universe, he needed a guardian for it. And who would be better than the assassin that was able to out-skill all who he came across?

If Sabre took the deal, the deity would grant him a kind of immortality where he will not age past young adulthood, will completely regenerate after receiving fatal injuries or his heart stops, and will gain extra strength in moments of need. However, he will be reborn after the amount of a human's lifespan has passed, and he must stay an assassin throughout every new life he's given. As a reward for his service, once all threats to the Apple of Eden have been eliminated and there will never again be a future threat to it, he will have earned his freedom to no longer have to serve the deity, will be able to travel through different worlds, and will stay immortal. If Sabre does not take the deal, he will be sent to the afterlife as normal.

Seeing the better than golden opportunity, Sabre agrees to serve the deity for as long as he's needed. Once given the power, his eyes turn a solid, glowing white.

Sabre spends countless lifetimes as an assassin, conquering any threat to the Apple of Eden and becoming a sort of legend of the world. Over time, his name becomes one meaning immortality itself to the humans. The evils learn to fear it, and the hidden eyes marking them as his next target.

In the final incident involving the Apple of Eden, Sabre is temporarily granted some of its power in order to stop the last threat. In the heat of battle, no one noticed a small offshoot of its power flying away, shimmering with life, into a rip in space. Sabre eventually won the battle, and with it... his freedom.

Now unbound to the deity, he gets one last sendoff from it as he goes through a portal, starting his new never-ending life as whatever he wishes to be.

Over time and throughout many worlds, Sabre learns more skills other than what he did as an assassin. Building, crafting, farming, fishing, mining, he learned it all. He developed even more of a personality, before then having to stay serious a lot of the time. He found hobbies and interests that he wouldn't even have experienced before in his old world. He became who we know and love now.

Soon, he reached a world familiar to us with a tomb nearby, and we all know the story from there.

Sometimes Sabre would teach the Steves how to throw or use knives in battle, or accidentally let a few old habits show, or use the reflexes he had gained from his old life, but other than that he never used any of the techniques or skills he knew as an assassin. He didn't want his old life interrupting his new one. Sabre decided that he wanted to remain in the Steve Realm, building his life to be calm and peaceful after the loop reset.

However, it couldn't last forever. His past eventually came to haunt him.

One day, a long while after the reset, ghastly portals suddenly start showing up around the Realm. From them, undead people emerge and begin to attack. After a lot of confusion, panic, fear, and many meetings between the Leaders, Light, and Sabre (who's there as a source of unbiased advice, and the Leaders have picked up of his strategizing skills), there's eventually a solid plan to confront all of these hostile individuals at their base, which they constructed at the same time they were terrorizing the Realm.

Guards and various color soldiers forming an army, Sabre, Time, and Light included, they approach the enemy base. Having not been able to see them for himself, Sabre realizes something about the undead hostiles as they start to gather to oppose the army.

All of them are the same people he had fought as an assassin, still with the wounds of the killing blows he had dealt to them. Before, they wanted the Apple of Eden.

Now, they want revenge.

Alright, now that the cinematic stuff is over, here's some details:

- If the deity is ever to show up again in the story, Sabre will always bow to it. Not out of authority, as he doesn't fight for it anymore, but out of respect as they had become good friends over the time that he had served it.

- Professor Red always nearly gave himself an aneurysm whenever he tried to figure out what Sabre is, as both his immortality and hopping around worlds altered stuff like his vitals quite a bit. You can imagine his reaction when he finds out that Sabre is fricken immortal.

- Sabre was the one that taught Light Steve how to fight, shortly after his fusion with the Light Crystal. Even during the story if this AU, Light has never been able to defeat Sabre during a spar, as Sabre didn't teach him everything he knows.

-One of the weird things Sabre does is use the 'pocket sand' tactic in almost every serious fight. He's hung onto that tactic ever since the earlier times of his old world.

- Technically, Time is also immortal, because... he can just turn back his age/undo his wounds and never actually die unless something stupid happens. So... immortal duo :D

- As soon as the secret of Sabre's old life is known, he starts not caring about people seeing his assassin abilities and skills. Suddenly, he's scaling walls like nobody's business and he jumpscares everybody because no one hears him coming. He probably gives his friends heart attacks by jumping off a really high place into a hay bale eventually.

- Many villains or general annoyances have gotten a few knives thrown at them. Mainly Shadow, Void, and Hypno.

Okie now situations:

—

(Sabre talking about Slicer when someone brings up how messed up he is.)

Light: So uh... all of these guys' deaths were your doing, right?

Sabre: Jeez, saying it like that makes it sound worse than it was. But yeah, why?

Light: ...Why'd you do that to that one guy apparently called 'Slicer'?

Sabre: Oh. Him. Uhh... I was really hotheaded back then, and he was seriously annoying. Like, for some reason he just didn't react to any kind of injury and was always cackling. Also he kidnapped a friend's sister, kidnapped a bunch of other people (including me), murdered a bunch of villages, poisoned me, aaand decided to make himself a threat to my friends.

Light: Okay I kinda get that, but... seriously, he's basically mutilated, he doesn't even look like a human anymore-

Sabre: Look, I'm not proud of that at all, man. It was a long time ago and I was admittedly pretty dumb back then. I dunno what else to tell ya.

—

(Sabre and Time hanging out in the house.)

(They noticed a small spider on the ceiling, and before Time can even react, Sabre's already thrown a knife and it perfectly splits the spider in half and sticks to the ceiling. Sabre continues the conversation like nothing happened.)

Sabre: Yeah, I have no idea what that guy did to tick off that sheep so badly. Seriously, it was dragging him all over the village-

Time: (Finally having snapped out of his shock) Wait- Hold on- wHA- WHAT.

Sabre: What? Did I talk too fast again?

Time: HSJSHAKSH- NO, THE SPIDER- WHAT WAS THAT??

—

(Sabre helping a few Guards sneak into the enemy base. They're all hiding behind a stack of boxes while waiting for the one enemy in the room to leave. Sabre gets impatient.)

Sabre: Hey, ever heard the sound of someone's head smacking on concrete while the vertebrae in their neck is being separated?

Guard: ...No-?

Sabre: (Extending his hidden blade with a chaotic grin) Would you like to?

—

(Shadow watching Sabre beat the living daylights out of the undead Assassin's Creed enemies from a very good distance so Sabre doesn't see him.)

Shadow: (Clearly terrified) ...How in the World Below did I not get absolutely wiped when I fought him?

—

Okay baiiiii