The demon king started:

"Even though you call us demons, that's not what we are."

"What do you mean!?" I asked.

"Well... It's a long story, so let me start by asking you: What is the source of magic?"

"Easy! Magic is the power within us all that lets us use magic items."

"OK. And what if I told you that this is not true?"

"What!? How is this not true!? It's pretty common knowledge!"

"Magic is powered by spirits. It is their power that you humans are using."

"And what are those spirits?"

"Well... They don't normally have a form, but we are an exception."

"What!? You are..."

"Yes! Our actual names are the spirits of darkness."

"OK."

"Uh... That's it!? Just an OK!? No reaction at all!?"

"No. You can continue."

"Um... Ok... Where was I? So, we are actually spirits of darkness, and as I said, unlike other spirits, we have a physical form. That's because our power far surpasses that of other spirits. For example, just that holy sword of yours probably has thousands and thousands of spirits of light's power just so you can have a slight chance of beating me."

"Wait... Other than spirits of darkness and spirits of light, are there other kinds of spirits?"

"Yes. There are a lot of different types of spirits. There are spirits of fire, of water, of wind, of earth, of growth, of life, of death, of power, of lightning, of wifi..."

"Wifi? What's that!?"

"Trust me... They're important... More than you think..."

"Alright... Are there more?"

"Yes! A lot more, but I'm not going to tell you about them all day so I'll stop there."

"OK..."

"Anyways... A bit more than 1000 years ago, humans discovered our existence. They were afraid of us at the start, so they called us demons. They waged a war on us and the king pretended their kingdom was being invaded. For that reason, they sent a hero. Along with his holy sword, the hero arrived here in our domain with the intent of stopping our supposed invasion."

The hero interrupted him:

"Let me continue from here. I realized, when I arrived here, that the king had lied and that there never was an invasion. I was furious! I then decided that my new goal was to defeat the real evil in this world, so I went back to the kingdom and pretended to have sealed the Demon Lord to be permitted access back into the castle. However, things didn't go exactly as I had planned... The king, let's say, wasn't planning to let me roam freely from the beginning... He probably figured that I'd overthrow him if I had an opportunity, so he had me put in jail before I could even get to him. I couldn't really do anything to the people that were taking me away because they were only following the king's orders and were simple civilians. It is against my morals to hurt people who did nothing wrong... Thankfully, I managed to get out thanks to a friend that I made there. I sneaked past security and arrived in the king's room, but he was waiting for me. I didn't have my holy sword with me so I was pretty much powerless against the man who was revered as the most powerful person in the kingdom. But he didn't do anything to me physically. Instead, he put a curse on me and let me go. This curse makes me immortal, but I also cannot kill. Because of that, I couldn't kill the king, and so I searched for my holy sword and stayed in the Demon Lord's castle for 1000 years, until recently. I decided that I should search for a successor who could defeat the true evil in my stead. That's how and why I met you that day."

"Wait! So you want me to kill the king!?"

"Pretty much..."

"I can't do that..."

"Then, I have another thing to tell you. I am immortal, but there is one thing that can kill me. And that thing if the hero slays me using the holy sword."

"What are you implying?"

"Either you defeat the king or you can slay me and leave my mission never to be completed."

"Can I just... Not do anything?"

"No."

"Why?"

"Because it's your destiny."

"That sounds super corny..."

"Yeah... It sounded way better in my head... Anyways! You wanna do it?"

"OK then..."

And so, my mission changed, for some strange reason...