Really good make another would u it was eccentric seeing the old team back together. Most importantly the dialogue. It helps establish that their friends

Hey, it looks like you have some good ideas going on here, but it's really hard to read and I wasn't able to follow it. You need to break up your paragraphs when someone starts to talk, or when the speaker changes.
(Example from your last paragraph):
Leaving the room, John could no longer hold in his question about that mark.
"Arbiter what was that symbol burnt into your chest?" John suddenly questioned out of the blue.
"Symbol?" Arbiter cocked his head, as they traveled down the huge empty hallway.
"The one of the right side of your chest." John replied, scorfully. Was the Arbiter playing dumb with him?
"This?" Arbiter partially removed a chest plate. Showing the deep burn mark etched into his flesh.
"Yes that looks really painful." John nodded. Judging by how deep the burn was, it looked like an act of torture.
"The physical pain can no longer be felt Spartan." Arbiter hung his head low.
"What is it?" John pushed.
"It is a mark of shame. Branded upon me for the destruction of the ring you destroyed. It symbolizes heresy, and dishonor."
Thel covered the mark back up. John took a moment, searching for something to say. Considering it was his fault, the Arbiter had this permanent mark branded on him. The best he could come up with was an apology.
""I'm sorry about that." John apologized.
(End Example)

Again, I think you have good ideas here and I like the character interactions, but you probably won't get as many readers as you should if it's hard to read in paragraphs like this.

God bless and happy writing!

ugh... why do people bring the arby back? he was in two freaking games. add someone else ffs.

Awesome story so far! Love the long chapters but I hope this isn't gonna be just a one-shot! Bit confusing at the start dunno if that was just me but PLEASE continue its awesome and I believe with a bit of improvement could become an amazing story!

I will say that the idea is interesting but there are several issues.

Pelicans aren't slipspace capable, and Sangheilios is no where near Requiem which means it would require him to have one.

Also, Master Chief is out of character in a bad way. He rarely speaks and would not ask the Arbiter about the Mark of Shame unless it was relevant to the situation, nor does he make idle conversation.

Some of the banter between Rtas and Thel doesn't seem...Elite-like. For example: "Thank you. I was only teasing brother". I don't think the Arbiter would or is capable of chuckling either, though I might be mistaken but I don't recall it from either the games or the books.

Lastly, if your new to writing, I suggest a beta, they can help with grammar, spelling and formatting. It would help make this interesting plot bunny more readable and enjoyable.