(Author's note: So this is my headcanon about souls, but I found it boring to just write a theory about it, so I decided to let Artorias and Ornstein explain it. Enjoy.)
"I am dragon slayer Ornstein, your future captain and I am here today to teach you about soul theory."
Ornstein inspected the fresh batch of silver knights that had made it through the trials. Some of them were murmuring. He could make something out along the lines of: "The captain himself is giving this lesson?"
"Of course, there is more to being a captain than just giving orders.", Ornstein said while looking in the talkers direction. "Now I would prefer for you to pay attention, cause I don't want to say everything twice. I suppose you are all familiar with the four great souls?"
A shy hand was raised: "Actually... how was that again...?"
"What do they even teach you in school today?", Ornstein asked and picked up some chalk to write on the board.
"So, there are four great souls who all have been picked up by different hosts. There is the soul of fire, which was picked up by our Lord Gwyn." He added a quick picture of Gwyn to the board and continued: "We also have the soul of life and the soul of death. These belong to the Witch of Izalith and Gravelord Nito, who both are our allies." He added respective pictures of them too. "And finally there is the dark soul who was picked up by... uh..."
By whom exactly? He couldn't remember. Did they even play a role in the war? Lord Gwyn was obvious. The witch of Izalith and her daughters had helped out with pyromancy and Nito had unleashed death and miasma. Ornstein counted them on his fingers, just to be sure. Only three. The first appearance in front of the new silver knights and he already was stuck. Luckily, he saw Artorias walking by. He called out to him: "Artorias, could you come here for a moment?"
The wolf knight strolled over to him: "What is it? Ah, teaching soul theory? Was it really necessary to add pictures of the soul bearers? They are really cute though." Ornstein lowered his voice to a whisper: "Please help me out, Artorias. Who exactly picked up the dark souls?"
"What? But Ornstein, you were there!", Artorias shouted far too loud. He could hear some of the silver knights snicker. "So were you.", he said still whispering. "Please help me out, I can only remember Lord Gwyn, the witch of Izalith and Gravelord Nito."
"Oh yes, the bearers of the soul of fire, life and death. Let's see, the dark soul belongs to..." Artorias stopped. "Um..." He seemed to consider it for a moment. Ornstein could hear some of the silver knights talk again: "Wait, the captain was part of the war? Wasn't that hundreds of year ago? How old is he?"
"232, maybe a few years older or younger, I lost count.", Ornstein casually answered which prompted the silver knights to take shelter behind their desks.
"I counted, it is 236 actually.", Artorias fell in. "Wait, where were we? Ah yes... the dark soul." After a while, he gave up: "I am sorry, I can't remember. I guess they didn't play a role in the war and were never introduced to us."
"Well, thanks anyway.", Ornstein said. "But what we know about the dark soul, all humans are descendants of it. Here in Anor Londo there aren't many humans around, but we get to this later."
"Now to the nature of souls.", Ornstein continued the lecture. "Everyone of us is having a soul inside them. It may even happen that they linger around after death. It is unknown why this happens, but it looks like this." Ornstein pulled out a soul from under the desk, showing the white lingering thing around. Artorias was taken aback: "You are carrying actual souls around for this lectures? Where did you get these?"
"Don't worry, Artorias. These are from the executioner." Ornstein and Smough, the executioner, hated each other, but he was a great source for souls from the executed prisoners. Ornstein had to listen to a lot of teasing when he went down there and asked for another one, but it wasn't like he could just go out thereand kill someone to snatch their soul.
"That astonishes me even more!", Artorias said. "You are going down there on your own free will?"
"Would you please let us continue the lesson, Artorias?", Ornstein said, growing annoyed. Why had the wolf knight even decided to stay? Maybe he wanted to stay around if Ornstein got stuck again.
"Anyway, before Artorias interrupted us.", he glared at his friend even though Artorias could not see it cause Ornstein wore his helmet. "If you come upon one of these, you can consume it. Like this." Ornstein popped the soul for demonstration and could feel how the power got added to his own. "The power of the soul will get added to your own soul. But it not only happens if you pop a soul. Every time you kill one of your foes their soul power will be added to your own. This can lead to you growing larger than you are used too."
One of the silver knights raised their hand. "Yes, what is your question?", Ornstein demanded.
"Is that the reason you and Sir Artorias are so big?", they wanted to know.
"Well, it is a bit different for us.", Ornstein said and then explained: "Both me and Artorias have a part of Lord's Gwyn soul of fire."
"How did this work? Did you pop it like you showed to us recently?"
Ornstein shook his head and Artorias took up the word: "No, it worked different. We used that soul part to fuse with our own soul."
"I remember how strange this felt.", Ornstein reminisced. "It felt like..."
"Like you just drank far too hot soup.", Artorias casually stated.
"I wanted to say like you had a hot, burning feeling in your chest."
"Exactly."
Ornstein could hear some of the fresh knights chuckle again. "I really thought they were far more professional.", he could hear one of them say and the answer: "I know, right?"
"I heard that. You two can come talk to me once the lesson is over.", he said coldly and pointed at the chatters, who looked very small after this announcement.
Artorias was amazed: "You are hearing everything."
"I gave lectures like this for over hundred years now. I had enough time to learn it.", Ornstein said not without a little pride.
"So, you silver knights here won't be in danger to grow as big as we are, but you can expect to grow a bit the more souls you absorb. There is also a technique that allows you to stop your growth, but it is really hard to master. The only one who managed to master it at once, was Ciaran." Artorias nodded and added: "Because of her duties as an assassin she felt she shouldn't be gigantic and mastered that skill extremely quickly. But now she often seems to be upset that she is so small."
"Well, she is only around the size of a human. That is really small.", Ornstein said. Humans rarely grew larger than 1,50 m and him and Artorias had started out as much larger sizes before their soul power had given them their final height.
"And one last thing about this. Your growth will also scale from your actual height. So, if someone who is larger than you acquires the same amount of soul power, the both of you will still have different heights.", Ornstein explained. "Anyway, it is even possible for anyone of you to gain a special soul. The more powerful you are, the more likely it is that you gain a special soul. But these are really rare, so we can't show one of them around."
"But they look a bit different, like they are a shining light.", Artorias said. "And you can use them to recreate weapons. It seems like the soul still remembers the most used weapon of their owner. Our giant blacksmith here knows a way to do this. So should you ever come upon a special soul, bring it to him."
"A thing like this is possible?", one of the silver knights asked.
"Yes, everything Artorias just said is true. For example, should I die for whatever reason and you get your hand on my soul, you could use it to recreate the dragon slayer spear."
The silver knights were murmuring again, amazed by this discovery. "But it is clear, that you can't go around and just slay people for their weapons. Besides, most of the special soul bearers are really really strong. You are more likely to lose your life instead.", Ornstein explained casually which shut the silver knights down instantly.
"And now we have to talk about the specialty of humans."
"You mean the dark sign.", Artorias said.
"Exactly. It is possible for humans to be cursed by the dark sign. That means they are undead. And that means, they can't be killed. Well, they can, but they will return after a while. The danger comes from them turning hollow. That is a state, where they lose their mind and attack everything on sight."
"You may be aware of the undead asylum", Artorias added when Ornstein stopped to catch his breath.
"Every undead has to be sent there. So that means, undead have to be captured. It is fine if you ask us for help if you encounter an undead, they aren't easy foes. And the nature of souls with undead is like this..."
"Oh, let me explain this part, please.", Artorias requested.
"Fine, go on."
"We told you about growth because of soul power. But the moment a human turns undead, they stop growing regardless of how many souls they gathered. The size of an undead is a good indicator how strong they were when they were alive, but you should be wary of even small undead. They can be much stronger than they look like."
"We don't know why this is the case.", Ornstein explained further. "But in theory, the souls try to fill out the hollowing process the undead go through."
He stopped for a brief while to let the silver knights take in the information before taking up the word again.
"So that was everything for today. Does anyone have some more questions?"
(Author's note: If you have further questions, just ask. Maybe I'll get into character and answer them.)
