Chief,Arbiter and Guilliman(No bodyguards just the 3 of them) The pilican that carried Chief and Arbiter and the Stormbird who is carrying  Guilliman on Board Towards the High Charity crash site to Rescue the A.I Names Cortana and Fight off the Flood and escape

The 3 begun to move inside to the High Charity but it din't take too long before they engage the flood coming to them by waves.

"It appears they know that we were coming" said Guilliman as he fired his bolter to flood-infested brute in the head.

"No doubt the grave mind have a set of traps for the likes of us" said the Arbiter as he decapitate another flood.

"But he will never expect the likes of Lord Guilliman" said Master Chief while shooting mulitiple floods with his Assault rifle. as he knew the Grave mind was targeting the UNSC and the Separatist while the Arrival of the Ultramarines Legion have change the balance of the War. 

The 3 of them continue their move they will not stop until they find Cortana. They searching each and every area they could find sometime Guilliman will separate with them since he is a giant and he cannot fit some door or bridge that are thin and could not hold his weight and he is force to find in his own.

The deeper they get the more and more and dangerous and more lethal flood they could find as they trying to stop them.

"This is getting to be fighting against the horde of Nurgle demons" said Guilliman as he swing his sword(Still not ignite) killing dozens of Flood-infected brute.

He looked at his companions they fared and could face as many as they can. some of the floods were using long-range weapons it was only their shields and armor that defend them against plasma and organics and bullets. 

Thought they  haven't many as if the flood have expected them to save Cortana each section they came across are all traps but it din't stop them to find her.

As they walking towards the bridge a tentacle suddenly appeared and swipe aside  Guilliman  hitting him hard making him fall from the bridge Both Master Chief and the Arbiter have called him name.

"Lord Guilliman!" they shout in unison as Guilliman was 800 meters below from them

"I'm Okay i will only find another way" he said as he stood "Go on search your friend Master Chief i'll find another way" he said as he looked at them 

"We will see each after we found her" said Chief as he looked at the Arbiter and he nob and they continue their search.

"That foolish creature he will pay when i see it" said Guilliman to himself 

"Then we shall see Lord Guilliman "Said a booming voice from his head

"You must mistaken me to be a food abomination" said Guilliman as he walk and he will not going to be break by this creature as he had face far worse than this creature. 

And not for long Guilliman face the flood they pouring out from every direction Guilliman readied himself for fight. His Hand of Dominion he activates and it glowing with power. 

Guilliman charge at the oncoming horde as there's no way to escape so he must attack at the front. Guilliman Fired his Bolter to largest and most dangerous flood host with each swing from his blade severed limbs, head and other parts of the body drop, while the Hand of Dominion sent them flying or destroying them completely.

After finish them off Guilliman continue his quest. and it take him long enough when he found the person they were searching for the one they called Cortana lying in somein the pillar(I don't how to describe) 

It was shielded he saw her lying Guilliman punch it but not before he deactivate his power fist then the shield destroyed.

And the two were facing each other and could clearly saw her face.

If Guilliman was a Mechanicus fanatic during the Days of the Great Crusade in M31 he would have destroy her without mercy but things have change when they discovered the Casticator Titan a fully fledged A.I. by the Machanicus and the Machine wants to serve Humanity once more and he even given them His STC at first they are wary at the Machine but he explains to them why they rebelled and it's because they were treated as tools of destruction by their creator.

and because of this the Tech-priest were enrage and he promise to the Titan that they will not be treated as such and when he return it to Mars it took them decades before they accepted it and it was approved by the Empress to build as long as they do not abuse it. 

And their fear of A.I. was slowly fading until they accept them and they even begun to become more automated.

She stared at him as if she is uncertain if he is here to rescue her.

"Are you here to destroy me? and what are you? And which factino do you serve?" she asked him 

"Are you Cortana?" he asked at her

"Yes, and you don't have answer my question yet" She said 

"I'am Roboute Guilliman(Removes his helmet and Cortana Gasp) Primarch of the Ultramarine Legion Lord of Ultramar and son of the Emperor of Mankind" he said as he stared at her

"Are you another experiment of Dr.Halsey? are you of another bunch of spartan? and who the hell name this Ultramarines it's more like Ultrasmurfs" she said as she cross her arm

"For your question lady no i'am not another experiment we are not another spartan and i'am the one who named them Ultramarine so don't call us smurfs whatever that means" he replied

Cortana resist to urge to laugh at the man comment "Is this guy seriously don't know the smurfs?" she though

"I'm here to rescue you Cortana along with your friend spartan the one they called Master Chief" he said Cortana was surprise of this

"Is he really here? if so he din't break his promise to me"

"Yes. we must Go" he said

"Ok, but do you have an A.I Chip?" she  asked 

There was silent for 2 seconds Cortana broke the silent "So you have no A.I Chip?"

"No."

But in his mind he is forming Theoretical and Practical 

"Theoretical: I have no A.I chip

Practical: I can let her enter to my armors system but she can saw all the records in my battles and the horrors from my galaxy. but she can be helpful"

Guilliman has made a decision 

"Take my hand Cortana" he said as he extented his hand

"What are you gonn-" before  she could finish her sentence Guilliman interrupted her

"Just do it Cortana"

She did what she just told and with it her form disappeared Guilliman put on his helmet and he saw Cortana's Face

"Wow, what kind of armor is this? it has an unknown metal and oh a whatever thickness this armor is i'm sure it could survive from a direct hit from MAC Gun or in theory a nuclear weapon" she said playfully

"And your armor has a massive energy inside of it it's much more massive than the Mjolnir armor" she added 

Guilliman begun to moved "And your A.I is quite primitive mister Guilliman"

"It's Lord Guilliman, Cortana" he corrected her

"Okay okay Lord of the Ultrasmurfs" she said playfully

"So we're are you from Lord Ultrasmurf which planet are you?" she ask and it irritate him

"How did the Master Chief could handle her?" he though

Guilliman was thinking but he could give her answers but not giving her the worst of nightmares he is avoiding her to go madness

"You will not going to believe this Cortana"

"Try me"

"Well i'm not from this universe"

She blinked then laugh 

"Are  you serious" she said still laughing 

"Yes, i'm quite serious Cortana and i will give you some records to my galaxy"

He then showed her his records of the M31st to M42nd(His awaking and post heresy(But no heresy)) but he did not give her the worst enemies

"Wow fascinating and this armor records shows OH MY GOD YOUR 11,000 YEARS! are you telling me that are a fking living Relic? how can they make you this old? Are you even Human?!" she asked as she shock

"For my age yes i'am 11,000 years old,i'am biogically Immortal, And i'am Human but i'm not even sure if i was Human i was created by my Father the Emperor of Mankind we are 20  genetically-engineered transhuman we were never been born" he said he still alert from his sorroundings 

"So you were clone or something?" she ask

"Maybe we were created to be the Perfect weapons while ours sons the space marines are our lesser copy but nonetheless we them as our sons, Our Father treated us as Weapon while our Mother the Great Empress treated us her sons." he finish

Cortana begun to scan the Primarch's biology and it surprise to her she looked at whole he is human a perfectly human but inside of him has more Perfect internal system by all mesurement extra hearts and lungs and possible immune to any desease  currently.

"Whatever inside this guy is, only one i know of, and it is power...a power i never seen before this guy is not just a super-human being experiment if my readings is right he has huge power source inside of him like he dwarfed the power of the nuclear weapon and should never be get into wrong hands especially the flood" she though  'This guy is a perfect living weapon"

"Now you understand me, Cortana" 

"Yes"

"My blood is holds power and i wish you erase all of your copy after this is finish" 

"I've already done that Lord smurf, can i call you that?" she ask

"You can call me whatever you want" he replied

As they continue to walk for about a 30 minutes Guilliman have attempt to cummunicate his companions but the signal has been jammed

During their walk Cortana was talking through his helmet keeping company and Guilliman had admit that he enjoys her company

"Contact" warmed Cortana 

Guilliman readied himself to fight and to Cortana's surprise he is far faster than Master Chief and even the fastest spartan Kelly and her speed is mere fraction of the Primarch

It din't take long to kill all the flood they begun to move again

As they moved They Entered into a very large hanger and is full of flood infestation

"Something not right in here" said Guilliman the door that he entered were shot off and then suddenly a large chunks of organic block his way.

"Lord smurf i think we are just Trap in here" said Cortana 

"Isn't it obvious" 

"Welcome stranger" a voice came

Then a gigantic flood appeared in front of him with large tentacles

"What is this Cortana" he speak her internally through his helmet

"That lord Guilliman(Serious) is the Gravemind" said Cortana

To Guilliman's eyes this is more like a creatures of Nurgle as he looks close to the Creatures he saw Tons of corpse around it's body 

"If you wondering what's body the corpse of all species it consumed both flood and non-flood it absorbed all of their knowledge and making the Gravemind" said Cortana

" You do not Belong in this galaxy stranger" it said "Give me this A.I. and i will let you leave if not you will die"

"I'am not afraid of you abomination you are not even suppose to exist" said Guilliman the sword of the Emperor begun to ignite as he channels his power to the sword.

Guilliman finally ignite the sword as well as the hand of dominion glowing with power as he is going to face a true powerful foe

Even notice a build up energy source from the sword

"What kind of power does the sword have? it not just advance and it is a mystery of how did he activate the swords flame" cortana's thought 

Guilliman attack the Gravemind. The Gravemind begun using it's tentacles to hit Guilliman but he dodge but they are many it even hit Guilliman

"Cortana do you have anyway that we can get out of here alive?"

"I think not Lord Smurf  he blocks all system and the only way is to fight him" Said Cortana

With that Guilliman swing his sword and flame burst out of it.

It hit the Tentacle making the Gravemind scream in pain

Cortana was surprise 

"The flames hurt Him" said Cortana "What kind of flames are you using Lord Smurf?"

"Those are not normal Flames Cortana that's a Psyhic flame, and it appears it's body did not regenerate" said Guilliman as he looked at the Creature

"YOU WILL PAY FOR THAT!!"

Guilliman attack the Gravemind again 

The Gravemind use all of it's tentacles to attack Guilliman each tentacles smash themselves to Guilliman which he dodge easily, Some of the tentacles are cut off by his sword or destroyed by his power fist, one tentacle lucky enough to hit him and Guilliman drop his sword

"Do you think you could reach your sword Lord Smurf?" Cortana ask

"Yes"

But before that Guilliman run towards the Gravemind Head(Mouth maybe?) As he Charging his powerfist 

He jump at the Gravemind Head and he hit it such as the force of the Primarch's strength as he able to knock the Gravemind.

Guilliman run towards the sword and he grab it and attack the Gravemind again who are screaming in pain.

Gravemind attack Guilliman with it's remaining tentacles.

One tentacle cut in half

"I'AM STEEL!"

He Then cut another

"I'AM DOOM!" 

He punch the gravemind making it fell to the ground

"I MARCH FOR MACRAGGE!!"

He jump atop of the abomination and he swing his sword and

"AND I HAVE KNOW NO FEAR!!'

A large flame burst our burning the Gravemind killing it.

"You have done it, you killed the Gravemind" said Cortana happily 

There communication came back

"Lord Guilliman were are you?" ask chief

"We're  here in the hanger chief" said Cortana

"Cortana is that you?"

"Yes, and let's get out of here right now" said Cortana

And with that Guilliman fight his way 

Not long after he met with his companions.

"Is that the arbiter?" Cortana ask was surprise

"Yes" Said Chief 

Guilliman extended his hand and he open his palm revealing Cortana's avatar

Chief took his Chip and he it to connect it to cortana now chief is reunited with his friend and begun to explain why the Arbiter is them.

"Let's go and get of here" Said Guilliman they all nob and Run to their transport

and Took off 

and go to 

Finish the Fight

(A/N:The Reason  why i pair Guilliman with Cortana it's simple i need make to become friends and know each other)