In the World of Sword and Magic as Space Marine

The Custodian had no answer. Thunder peeled above them, shaking the cavern and jarring loose a rattling hail of falling pebble dust.

'My king, what now? What comes next?'

The Emperor turned away, walking into the darkness of the cavern while the storm hammered the dead city so far above. He spoke three words that no Custodian had ever heard Him speak before.

'I don't know.'

From: Master of Mankind, conversation between Emperor and Custode

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"So, this is going to be our last meeting." Numen (Star Child) said to William as he sat in his floating canoe. 

William finally realizes that he is not in his bed as his consciousness left his body and is now in the realm of the divine. The last thing William remembered was him going to bed and attempting to go to sleep as he was nervous for the next day's departure to space. 

"What do you mean this will be our last meeting? I'm sure, I'll see you when I make it to Imperium space." William said as he adjusted his chair. Unlike previous times, William was much bigger than before and the chair wasn't much of a comfortable as it looks. The golden chair that could be used as a royal family back on Earth, the thin-legged and normal-sized chair didn't give much room for a man like William who has the body of the Astartes. 

"That was the original plan until something unexpected happened after our last meeting. Now, I'm not really sure if we're ever going to meet again. Most likely, this will be our last time seeing each other." Said the Star Child and with a flick of his hand, he replaced William's golden chair with the Adamantium throne big enough for Astartes such as William. Made from the strongest metal known to the Imperium, the gold-covered throne had a symbol of the Imperium eagle on top while the bottom of the throne was one solid block that could support the weight of the Astartes. 

"Thanks, that is much better. Can I keep this?" William said but before he could get the answer Numen flicked his hand once again and a table full of coffee cups and tea appeared in front of William. With everything set and done, Star Child descends from midair to face his 'Not a biological descendant.' 

"Make yourself a drink. This will be a long conversation." After two cups of coffee were made, the conversation continued.

"So, what happened after our last conversation? If I remember correctly, you wanted me and Rana to have a child, a boy." 

"Yes, that was the plan, your son, Henry, right? Great name by the way. He was supposed to be my new vessel as he fits the requirement." 

"Ok, I mean I know he is special since he is a natural born between Astartes and Living Saint. Still, he can't be that special like Primarch," said William as he drank his cup or rather a mug of coffee. Star Child remained silent for a few seconds which made William finally realise something he missed until now and it was about his baby son. 

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"Oh, you gotta be fucking kidding me. Don't tell me you planted something in your DNA within the Grey Knight gene seed." William put down his mug and shouted with shock in his face. 

"Of course I did. Why do you think I allow my genes to be used to make superior Astartes? Just in case I made sure that I could unlock my own DNA code beside my own body to create the next generation of Astartes or even a Primarch. You and Rana happened to meet my time of need to create a new host." Said Star Child as he drank his own cup of coffee.

"Did you just say Primarch?" William was flabbergasted as he didn't expect this level of revelation to come from the Star Child. His son, blood, and flesh could be a Primarch. William continued. 

"That is not possible. The Primarchs were made from genetic engineering, not from natural birth." 

"Not exactly. You are right, my sons were born from my Primarch project through the lab but I did learn a few new things afterwards. Before my ultimate entombment to the Golden Throne, I implant everything I learned from the previous Primarch project and new into my very own DNA. Think about it, you have my genetic information and with your copulation to Rana who possesses traces of my psychic power meet all requirements to bring a worthy vessel in the end." Star Child explains everything to William. Small apportion of the Emperor's plan for the Imperium in case something goes wrong.

Star Child summons a baby monitor-like screen with Henry on it. William knew this wasn't a screenshot or record event but live footage of his son who is sleeping in his baby crib right now. 

"I want you to use an appraise skill on Henry. You will understand." Said Star Child. Hearing this, William activates his appraisal skill to see his son's status.

William did the appraisal of Henry before. At that time William saw nothing but a standard description of his son's name and who his parents are. There was nothing special nor skills of a typical protagonist such as William's, until now. 

[Henry Bassilo Nullinanis: Son of William Nullinanis and Rana Bassilo Beckett. The next generation and natural born Primarch. Former vessel of the Emperor of Mankind as Emperor made a decree to take another host, replacing the vessel of his choice. Born from the bond of Chapter Master of the Blacksouls who have a direct heritage to the Emperor of Mankind and Living Saint of the Imperium, Henry was born to be a new vessel of the Emperor but due to a change of destiny, the host is no longer selected to be a vessel for the Emperor's soul.

Ability: Appraisal, Inventory, Master of All Tools, Redacted, Redacted, Redacted, etc.]

'The next generation of Primarch. A freaking Primarch.' William read his son's status three times before his eyes left the screen. He couldn't believe that his son became the next Primarch and he didn't know it. No, he was denied seeing the truth until now.

"You hid this from me until now. Why?"

"Because I wanted to give you a time of peace when your son was born. One thing I learned from my previous mistake was that I took it for granted when I made my 20 sons and tools to achieve my ultimate goal. You needed time to see your son as who he is, not as another weapon to eliminate your enemy." The Star Child explains his reasoning to William in which he shows the peaceful moment where William spends time with his family. For a moment, William smiled and engraved all the images into his mind but eventually, everything must come to an end as William returns to the present time with the reality of the situation coming down on him. 

"You are right. But now that you are showing my son's true status means that something is coming." William said to Star Child and placed his hands together to get serious. 

"Yes, you are right. With Henry no longer my vessel and free from his original destiny, it means even I don't know what will become of him in the future. It is up to you to make sure your son becomes a man you would be proud of."

William heard the truth and went into deep thoughts but before he could make any decision, Star Child added. 

"You can't leave Henry and Rana here in the New World. The agreement with the other gods was to take all of my followers back home. Except for Cain's daughter since her mother isn't my follower."

With the situation locked into no other option, William finally opened his mouth to speak about his next decision in his mind. 

"In that case, I will place my son into a warp-protected pod while traveling through the warp. I don't know what will be on the other side of the warp portal but I will do everything I can to protect Henry and I'm sure Rana will do the same. Numen, I know you said this will be our last meeting but I hope I will see you once again on the other side." Said William as he stood up and extended his right hand out. 

Numen paused for a second before taking William's right hand and shook it before taking it back. Before William is about to return back to his real body, Star Child speaks as if he remembers something. 

"My son, remember to hold on to that piece of my light close to you." 

"What do you mean by that?" 

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William woke from his bed as his golden eyes saw the ceiling. It was still nighttime and dark but with small lights spread around the sky, blinking as if it was alive. Next to William, Rana lay on the bed with her chest rising and falling slowly with each breath she took. With silky skin and nightgown, Rana didn't look like a warrior saint princess but rather William's love of life and someone he would love to spend the rest of his life with. 

Not far away from the bed, William's son, Henry sleeps in his baby crib in peace as if no evil will ever approach him or harm him. The room was at peace and no amount of battle adrenaline or glorious moments in the war could replace this moment from William.

'I have work to do in a short amount of time.' William thought to himself and quietly got up from his side of the bed. After getting his clothes on and double-checking his wife and baby are in sleep, he opened the door and left in silence. 

William's destination wasn't that far from his bedchamber as he entered the room a few minutes later. With special locks and access only available to the Chapter Master, the room itself was fully sealed but could be transported if permission was given. Inside, the room was made out of white tiles with no sign of any gaps but was able to generate visible light for William to navigate.

There is no table or anything except for one thing in the center of the room. An advanced machine is beyond anyone's imagination as it can produce anything as long as the user wishes for it. A machine that can solve any problem given and is able to bring it to the physical world after going through countless calculations. In front of William is an STC (Standard Template Construct) and its main core, the brain of the machine itself. 

The machine came to life after a quick scan of William, identifying the person in front of it, and began to expand itself like a flower in the morning dawn. 

[Hello my master William, how can I assist you today?] The machine spoke with the human emotion of greeting as the avatar of the Mother appeared in the room standing next to the machine itself. 

"I want to make a few things before we leave today. Let us start with a transportation pod for a baby."

[What are the operation purposes and utilization?] The Mother asked. At the same time sense something within William that only she recognized but didn't bring it up as it didn't meet the current need.

"To protect my son and ready him for everything that can be thrown at him."