In the world of 40k, the Imperium of Man faces eternal war against the forces of Chaos and its followers. Once the servant of the Chaos is slain, such as daemons and traitor Astartes, their souls will return to their master's realm and go through the process of respawning. Based on how they were defeated, the time it takes to complete regeneration will be various, also there is a factor on if the servant has favor from the Ruinous Powers which will greatly help them recover.
The only real way of killing servants of the Ruinous Powers is few and rare to accomplish this. One way to truly kill them is to use ancient artifacts that can kill not only the physical body but a soul as well. The Emperor's flaming sword is a good example of delivering true death to the daemons and a named Chaos Astartes. Currently, Emperor's loyal son, Roboute Guilliman wields his father's sword and leads The Indomitus Crusade to reunite the divided galaxy while facing his traitor brothers on the battlefield.
If the ancient artifact is hard to obtain, then the next option is to have a person who is Untouchable or more commonly known as 'Blank' ability. If you are next to a person who has the blank ability, it will be very uncomfortable, and most likely wants to leave right away, but this same ability will save you from the enemy who possesses psychic power in the battle. As a blank can nullify every psychic power in the area and psychic powers are linked to Warp, it gives a rare opportunity to permanently deliver true death to the servants of the Ruinous Powers. This is why Emperor has an army of Sister of Silence who all possess blank genes and work next to Custodes to protect the Imperial palace from the forces of Chaos.
With Ruinous Powers having a vast amount of souls and the ability to respawn countless soldiers, the Imperium of Man faces a never-ending war against Chaos. From simple servants of cultists to Greater Daemons of Chaos Gods, Imperium of Man needs every help they can get and the Emperor sends his greatest avatar to help humanity. Enter the Living Saint, the servant of the Emperor of Mankind and living symbol of the Ecclesiarchy.
Over the millennia, Imperium had many saints who are extraordinarily and some were simply appointed by the priests from the Ecclesiarchy to boost the morale of the people. Saints who were appointed by the Imperial Cult can anyone who achieve greatness for the Emperor and Imperium such as turning the tide of the battle against forces of Chaos to becoming more than the hero of the Imperium.
Unlike Saints of the Imperial Cult, Living Saints are very rare and almost always are a woman (Mostly from Sisters of Battle). When Living Saint appears on the battlefield, it will literally turn the tide of the battle from near loss to victory as mere presence itself will boost morale, turning Emperor's servants' broken will to iron. Living Saints are Emperor's avenging angels and have the ability bestowed by the Emperor to deliver his judgment to his enemy. Carrying their own flaming sword and massive wing, Living Saints are the Emperor's miracle and his Greater Daemon against Chaos Gods' Greater Daemons.
Yes, Living Saints are Emperor's Greater Daemon according to the majority of fans of the 40k games, Living Saints are the physical presence represented by the Emperor after bestowing his power to his most loyal servant. For example, when each of the Chaos Gods brings out their Greater daemons, they each show a physical and literal representation of their master in battle. Khorne has his Bloodthirsters who are a literal powerhouse, and Nurgle has Great Unclean Ones who bring out all kinds of plagues. So, based on this logic, Emperor who was clade with golden armor and a flaming sword during his prime would have a Greater Daemon of his own by using his psychic power to create his own version to help humanity.
Overall, Living Saints are the physical embodiment of the Emperor's power to help mankind as well as one of many tools to complete his secret plan for the world. Of all Living Saints, Saint Celestine is a great example and the one who currently represents the 40k world as a Living Saint. During the Fall of Caida, she and her Geminae Superia participated in the fight against Abaddon the Despoiler's 13th Black Crusade. After the Fall, many of the survivors embark on the Celestinian Crusade to Ultramar, ultimately becoming a key member of the resurrection of the Roboute Guilliman.
Flashback to Star Child's Golden Realm,
"First of all, why aren't you guys hit the bed yet? Sooner or later I'm going to need physical body and you guys need to hurry up." Star Child quickly finish his last words before Rana could process the last part. Right away, Star Child continued to talk and open a case of beer on the table and pass one to Rana, she took the beer can but didn't know what to do with it
"It is called beer. I got the idea from William and decided to make my own brew so it should be pretty good since I made it. As I said, you have that white feather that can grant you any wish. So, what about it? I can send your soul to another world with all your memory intact." Star Child open his beer can and took a sip from it and wait patiently as Rana quietly stare and hold a cold beer can. Finally, she spoke with excitement as if she figure something out.
"Can you send me back to my world? After all, it is a wish so I can go back to my own body right?"
"Nope, can't do that since your body is literally filled with 14th legion's poison. Even if I send you back, those Death Guard's poison already did massive damage to your body. You have no healthy body to hold your soul once you go back down there. Sorry."
Hearing this, Rana got up from her seat and put down the beer can on the table. Refusing to believe that she can't return to the mortal realm and is forever lost to seeing William again, she returns to her old self with fire in her eyes.
"That's not good enough! You said I can have any wish and I wish to return to my old world. Not another world or your world but MY world so I can continue my fight next to him!" The room echoed with Rana's voice and she stared down at the man who is still dim with white light as he took another sip from his beer.
"Well, aren't you a firecracker of a woman? You remind me of my old lady friend, Erda. I swear, once she put down her feet not even I can move her to change her mind. Me, can you believe that? Well, back then I was arrogant and had no patience so what can I say?" Star Child sigh after taking moment to remember his old life and the mistakes he made including his fellow perpetual who left him due to his radical decision and plans for mankind. He continued.
"You know, I never meant any of this you know. It is just that there was no other way for me to help humanity without becoming extinct races before us. I really did look for other options but there is not much I could have done besides playing the part on the large scale of this Great Game. What I'm trying to say is that I can send you back, but you will be part of this and you will never escape from it. Right now, you are my follower but your soul belongs to this realm and deities. I could have sent you to another world where there is no Chaos, Imperium, or all these crazy shenanigans but if you truly wish to return, I can grant you that. For a price of course." Star Child said to Rana and finish his beer and crush the can with his thumb and index finger.
"I will pay any price even if I have to sell my own soul." The declaration from Rana was good enough for Star Child to take a deep breath and put his hands out.
"Take my hands and I will do all the work. Remember, there is no takeback after this. You will become part of this once I finish the work." Just before he could finish his sentence, Rana took both hands and prepare herself for anything.
In a few seconds, the entire room turns bright gold and white light as Star Child began his transfer of power to the soul of the mortal being. At first, Rana didn't feel any discomfort but soon, the pain became unbearable as her stomach glowed white. The same area where Night Lord's power sword stabbed began to heat up and eventually spread all over her body, consumed by the Emperor's light.
"By my name and authority as the Emperor of Mankind, I grant you the title and power of the Living Saint. From now on, you will be known as Saint Rana Bassilo Beckett of the Lioness of Holy Terra." Similar to William's transformation to Emperor's Champion, Rana also embraces the light, transforming her entire body and soul into something else. In the end, Rana's body reshaped into a golden sphere as her soul became something beyond mortal, and Star Child held her soul in his right hand.
"Such a fine woman you found yourself, William. I do hope both of you will endure the challenges ahead and deliver some salvation to my Imperium. They are in need of some help right now." With that, Star Child approaches the window and opens it. Looking through the open window, Star Child saw the mortal realm below and with one swing of motion, he throw Rana's new soul down like a baseball.
'Oops, I should have given her a list of names I want for their baby to have. Oh well, I will give it to her next time. Now, what to do with this incoming fleet from warp portal.'
End of flashback.
"Just fire your weapons! Kill the source of the light!"
"I can see! I truly see the light of the Emperor."
All of the Night Lords are blinded by the golden white light and some even challenge themselves to see the light directly. The Emperor's light did more than blinding the traitor Astartes, it literally burn their faces and exposed skins through broken armor plating as they began to smoke and burn alive.
"Slay them, Sisters! Push the enemy to the void they came from!" Living Saint's words are more than enough for Sisters and everyone else to push forward with vengeance in their eyes. Those who lost hope and are low on morale are now high peak while the wounded are fully healed to return to battle.
Soldiers stand tall with weapons in their hands while the remaining knights join the fight once again. "Charge!" The single command from Captain Silvi Greenheart signals everyone to attack the enemy moment ago was spreading fear and terror. Everyone towards the enemy with whatever they can use as a weapon. Even the artillery crew and the rest of the support members step forward with makeshift weapons to kill the black knights who killed their friends and comrade of arms.
Sisters didn't hesitate to miss the fight. Sister Superior Sabina Mire took command of two Orders and was the first one to attack the Night Lords before Captain Silverheart gave her order. The sound of bolters, flamers, and chainswords roar the battlefield as Night Lords fell left and right. When Rana and the rest of the soldiers join the fight, it turned into a complete blood bath.
Rana's new great sword is powerful enough to pierce and cut the power armor but this didn't end as she also use her golden shield to bash the traitor Astartes's helmet and even decapitate the body using the edge of the shield. During the fight, one of the Night Lords managed to hit Rana with his bolter but the bolter round simply hit the armor and fell to the ground like it was nothing. After finding the person responsible for hitting her, Rana extends her great sword's hilt into a long spear and throws it with all her might. The Night Lord couldn't react in time to dodge the spear. When the golden weapon hit the traitor Astartes, it went through his chest and three other traitors who were unfortunate enough to stand in the way.
It was at this moment when Hellbrute bull rush ahead with no concern for anyone including his battle brothers. Seeing the incoming giant metal brute, Rana extends her hand to recall her great spear sword back to her hand and uses her golden shield to block the incoming Hellbrute.
When nearly blinded Hellbrute and Rana's shield met, Rana, use her full strength to lift her shield, pushing Hellbrute over her and slamming it into another side. Hellbrute fell hard and was unable to get back up as it was lacking physical movement to lift itself from the ground. Like a turtle on its back, Rana took the opportunity and lift her spear up high. Rana strikes down Hellbrute with her spear as the golden blade pierces the dreadnought tomb chamber. The golden blade hit former Captain Naralor, ending his life and the misery of Night Lords Hellbrute. When Rana pull her spear back, there was no explosion or last word from the dead but just leaking mechanical fluids and the smell of Chaos corruption.
Standing on top of Hellbrute and mighty, Rana saw the rest of the Night Lords fall from the battle as Sisters and soldiers take them out. Raising her weapon up high and emits another wave of bright light. Sisters, soldiers, and the rest cheer as they also raise their weapons to join the victory.
"We march to end this conflict and we will endure any challenges ahead. Now, come with me to victory and for peace." Living Saint declares victory speech and points her spear in the direction where they are planning to meet the rest of William's forces. The great battle of terror ended with a terrible loss but in the end, the Sisters and the army of the White Lion emerged as victors of the battle with help from the Living Saint, once a mortal woman, now a living embodiment of the Emperor of Mankind and proof that Emperor was watching over them.
'I can't fall from here. I must rejoin with others if that is the last thing I must do.' Captain Taradreel and two of his bodyguards managed to run away before the rest of his company fell to Emperor's so-called Living Saint. Even though he lost all his forces, it was enough for them to be a distraction and an opportunity to save himself from that traitorous light.
"Where and what do we do now, captain? We are no longer a company or even size to be claw (Squad). One of his bodyguards asks his captain as another one next to him show similar concern for their current situation.
"We will regroup with the other legion companies and report this matter. I hate the idea to join a different legion since we all decided to take our own territory but this changes everything. We are going to meet the 14th legion Warband at the fortress for the final battle." Captain Taradreel said to his two remaining battle-brothers and turn around when something unexpected stood in front of him.
Standing a few feet away was a tall figure made out of metal and holding a long staff. If Captain Taradreel's memory serves right, he was looking at Necron Overlord and it looks like he was in a battle of his own as his body was heavily damaged.
"Kill that abomination!" Taradreel orders his men to attack but it was too late as Trazyn the Infinite simply tosses his Tesseract Labyrinth to catch the last three Night Lords of 31st Company.
[Interesting turn of events. I was expecting to see Emperor's Champion but it looks like I found an interesting specimen to be in my collection. Saint Rana Bassilo Beckett, a Living Saint and lover of Emperor's Champion, this I have to collect once I get rid of pesky Death Guards.] Trazyn picks up his Tesseract Labyrinth from the ground and put it away as he slowly walks away, heading towards where William's entire forces are planned to meet.