Space travel in the 40k universe is complicated and very dangerous. The current Imperium heavily depends on warp travel to reach the other side of the galaxy as it is the fastest but most dangerous way to do so. As Imperium based its technology on pieces of the Dark Age of Technology, many of the ships the Imperial Navy used are nothing but old, outdated, and irreplaceable.
It is possible that old humanity during the Dark Age of Technology might know or possibly possess the FTL but there is no record or known tech to prove that theory, currently. One thing is for sure, the only way the humans managed to achieve a galaxy-expanding empire was thanks to one particular mutation within humanity's history.
Navigators are Imperial-sanctioned mutants who possess the Navigator Gene. Their unique mutation gifted them with the ability to see through the warp and even communicate with each other light-years away from each other. As navigators, they have the ability to see through the warp and find the right destination by using the Emperor's psychic light as the northern star.
The third eye or psychic eye of the navigator helps the navigators to maintain their sanity compared to regular humans. For this reason, they are one of the most important factions within the Imperium to maintain the eternal gear of war in the 40k universe.
As long as Imperium existed, minor factions and noble houses managed the Emperor's territory on his behalf. The noble houses of the Navigators on the other hand are one particular minor faction that is essential to the Imperium and its survival. Just like Adeptus Mechanicus provide war machines and the war effort, the noble houses of the Navigators provide the supply of navigators for the Imperium.
From ship navigation, communication between planets, interstellar transportation, and commerce, the navigators are the most protected and valued individuals within Imperium. If entire noble houses were to be exterminated by the Inquisition because they are mutants, the Imperium will once again fall into something similar to the Age of Strife.
Lastly, the navigators' third eye grants them the ability to not only see the power of the warp turbulence and storms but also the power to kill a mortal human. If feels threaten, the navigator can use the third eye to expose the mortal's mind with warp, killing the individual in an agonizing fashion and torment of pain.
Overall, the navigators are Imperial-Sanctioned mutants that even Black Templars respect even though they are mutants. This is because Black Templars see the navigators as holy mortals who can see the Emperor's light and witness his greatness every day. The Imperium within its limit tolerates psykers and mutants as long as they serve and are useful. It is unknown how old humanity used navigators to travel between stars or achieve interstellar communication but one thing is for sure, navigator houses survived for tens of thousands of years during humanity's darkest hours and they will continue to survive the current 40k timeline.
----------------------------
"Take over the command bridge, kill anyone who attempts to stop you, and take the ship above the atmosphere if you can. I will not have this plague ship infect this world." William gave his order and pick half of his honour guards to follow him to the navigator chamber. With his helmet console command, William took the quickest way to the destination with the weapon ready. Knowing that navigator is one of the important pieces of the ship, William expects the chamber to be guarded.
'Can't have the ship enter the warp while we are here. I am also a bit curious about how much the navigator is corrupted by the forces of Chaos.' The thought made William imagine what kind of mutation the plague ship's navigator had gone through. William knew of the third eye and how even the 1st Claw of the Nightlords avoid their eyes before firing their bolters on the corrupted navigator in the Nightlord's book.
"Beware of heavy guards and pox minions of the Plague God. Kill all and maintain within the blank radius."
"Yes, Chapter Master!" The guards respond to William's warning and made formation to push forward.
Unknown to William, the plague ship's navigator passes the command to virus bomb the entire planet which will initiate the launch in 30 minutes.
----------------------------
15 Minutes Later, Captain Auguste of the Honour Guards.
"Captain, the breach is ready on your order." One of the Honour Guard reports to Captain Auguste and with a single nod from him, the doors to the command bridge were breached.
The explosion of the metal doors knocks down everyone on the other side of the doors. Armed with auto guns, the command crew of the plague ship attempt to hold the command bridge by making the barricade. Unfortunately for them, the explosion and flying pieces of the metal doors shatter half of the ship's crew and officers before they could pull their trigger.
Immediately, Captain Auguste and his men enter the command bridge with weapons hot. The sick and mutated crew of the ship quickly fell as their physical strength was no match to this unknown Space Marines. Some even attempt to attack intruders with melee weapons but it was a pointless effort as power swords of immense power put them down like sheep in the slaughterhouse.
"For the Plague God and his eternal love for life itself!" The shipmaster (Ship Captain) shouts without much effort and fires his laspistol. The last remaining crew and officer also stand their ground with all their might next to the shipmaster as they continue the rate of fire with auto-guns and lasguns.
For all their lives, they served the ship and Death Guard legion with up most loyalty. They are the original command crew of the ship and are forever bonded to serve as long as they must.
They were there when Warmaster Horus declare decoration of war against the false Emperor.
They were there when Typhus changed the course of the fleet to be gifted with Plague Father's gift. Forever changing their lives and destiny.
They were there when Mortarion moved his fleet to the Eye of Terror, to claim a world within warp and rule as the rightful ruler without the light of the false Emperor.
They were and are now forever lost to the Nurgle as their souls can't return to the warp, ending the legacy and stories that won't be passed on to others to remember.
"The command bridge is now ours. Burn the traitors' bodies and move the ship to the upper atmosphere." Captain Auguste put away his sword and bolter pistol while giving orders. Just as the captain of the Honour Guards thought it was over, one of his men, the technician of the group spoke with urgency in his voice.
"Captain. The ship is currently preparing to fire virus bombs onto the planet and we don't have control of the fire sequence."
"Who is controlling the command of the exterminatus?" Captain Auguste maintains his commanding tone as an uncontrollable situation doesn't require panic or uncertainty. Panicking only will cause more uncertainty and loss of control of the situation. The best thing is to find the solution before it is too late. After a few seconds, the battle brother replies back with a much more calm voice.
"The navigator of the ship transfer the command of the ship during our last battle. He gave the order to the scattered crews to load the virus bomb."
"By the Emperor's light… Notify the Chapter Master of this situation. He must know of this before it is too late. All we can do is change the trajectory of the ship while ascending to the empty void. Even if the virus bomb is released into space and detonates, the virus won't survive without oxygen." With the contingency plan made, the command bridge team of the Blacksouls moves the ship, ascending higher and higher.
----------------------------
10 Minutes before the launch of the virus bombs,
"Open fire!" William's voice echoes through the big hallway while his guards fire their bolters, killing the chamber guards outside of the entrance. The overall trip wasn't that much difficult as William's men did most of the work with bolters and blades. All William did was fire a few rounds of plasma shots and threw grenades to purge the heavier units blocking his way.
'Way too easy. Too easy, if I have to say.' William thought to himself and immediately regret it as he received the vox message from his bodyguard captain. Accepting the report, William caught up with the situation on the immediate exterminatus using the virus bombs.
'Damn it, I knew I jinxed it.'
"Move away from the door, NOW!" William orders his men to open the path to the chamber door. Quickly obeying the order the Honour Guards of the Blacksouls move out of the way, clear away from the incoming William's fully charged power hammer.
Bang!
William's weapon of choice broke through the metal doors, only for William to be the first to be surprised at what was inside the navigator's chamber.
The navigator's chamber was damped and humid according to the armor's sensor. That was the lighter news as the whole room was filled with green plants and living organisms with eyes moving around. There are few light sources to show the important section of the chamber as the center of the chamber has a water tank that is supposed to be filled with nutrient liquid for the navigator's meditation.
Instead of nutrient liquids of blue color, the glass tank is filled with dirty green swamp-like water and insects crawling around it. Once mortal human and navigator of the Death Guard ship reside in the tank as a mutated blob of green filth with multiple eyes and a huge body that can't imagine getting out of the tank.
The creature's eyes and William's met the moment he entered the chamber. The creature was ready for him and was ready to kill the intruder without a second to waste.
'Shit, I mustn't see the eyes.' William turn his head around immediately and closed his eyes. To his surprise, he didn't perish or feel anything wrong with him, confusing him more and more. The confusion also spread to the other side as the blob navigator couldn't hold his shock and yelled out in panicking tone.
"Why aren't you dead? I saw you and you saw my eyes, you should be dead with overwhelming images of the warp itself." The mutated monster of the navigator said out loud to make sure his victim know what should have happened.
'What?' William open his eyes and turn his head to see the once mortal just like he was. The truth was tested and William saw the creature to which he didn't die from the third eye. Realizing that he is not in any harm, he raises his combi-bolter and approaches the creature.
"You! Withdraw the exterminatus order right now or I will turn you into a honeycomb." Stopping a few feet away, William activate his bolter and missed the creature on purpose to prove his point. Looking closely, William realized the creature is either stuck or become one with the liquid tank due to his oversized body. That means the navigator of this ship for 10,00 years (or less) never left this room as he is incapable of doing so.
"You will now do such a thing. Even though I don't know how you survived my third eye's glare I hold all the cards in the end. If we must lose, you will also lose for only Nurgle's blessing will be our salvation." The blob creature spoke with a cringing laugh while moving his small hands.
"You piece of shit! Do you intend to destroy the New World? What do you care about the conflict between Imperium and the traitor legion? You are a navigator who was supposed to guide through the Emperor's light."
"I turned away from the corpse Emperor's psychic light long ago. I only see the wave of the warp and the true God as his intention is pure and eternal."
"You traitor!" This wasn't from William but from one of his men as one of the bodyguards fired the regular bolter round, removing a large chunk of green flesh. The wound didn't heal and the creature screamed in pain as for the first time in his long life, he felt an unpleasant feeling of pain.
"How are you doing this? How are you all able to live after seeing my third eye and inflicting pain on me?"
Hearing this, William realized how stupid he was. His blank ability was still actuated and he completely forgot about it as he was more focused on fighting and killing traitors. For him, the blank ability is like breathing and blinking his eyes as if it was natural to him.
'Damn, I'm such an idiot,' With that thought, William realized how much the mutated navigator is suffering from the bolter wound and show an uncomfortable expression on his face. The psychic nulling effect indeed made the creature uncomfortable and even receive psychic damage from William's presence.
'If he feels this much from me, how about from the Emperor himself?' A new idea came to William's thought in which he decided to test his theory.
"Everyone, turn away from this creature until I tell you otherwise." William's order was obeyed and after confirming all of his men are turn their backs, he pulled out one particular item from his inventory.
Holding in his hand was a golden orb of the Emperor's light and possibly a horrifying thing the fallen navigator could possibly see directly. Unable to run away or adjust his head from the glowing light, the mutated navigator screamed in pain as all three eyes of his burned like embers.
"Behold! If you turned away from the Emperor's light, I will just bring his light to you. Now, do as you are told, or else I will turn you into a crisp green blob that not even Plague God will touch."
----------------------------
For defeating Death Guard Terminators:
[Blacksouls Storm Speeder Thunderstrike x5: These hovering speeder of the Space Marine vehicles is best for scouting but capable of carrying only two Space Marine. Armed with multi-missiles pods and lascannon, the Thunderstrike can deliver decent fight power to harass the enemy from approaching their target.]
For defeating Death Guard Deathshrouds:
[Blacksouls Devastator x 15: Centurion Devastator are long-range specialists and heavy weapon units armed with chain-linked heavy bolters and mounted missiles to support the frontline. These walking tanks are power armor of the Astartes who are already equipped with standard armor, making the Devastator a power suit more than power armor.]
For defeating Captain Kallgar of the Death Guard:
[Blacksouls Thunderhawk x4: Possibly the only way for Astartes to travel around the sky and space in style and comfort. The Thunderhawk is a multi-purpose aircraft of war for the Space Marines and Sisters of Battle as it is capable of quick deployment and possesses triple engines big enough to break the atmosphere. The aircraft is equipped with a large number of weapons from lascannon to missiles and enough defensive measures and control systems to fight off regular flights in air combat. The aircraft is big enough to carry 30 Astartes or 15 terminator units without worrying about the weight. As for the Dreadnought, the ship can carry 6 in total (Five Space Marines per one Dreadnought).]