Unfortunately for Death, he did not see any panic, dread, or terror from Edward's face; just a plain and absolute calmness. He wondered whether this guy did not care as much as his parents as he let on.
"If you do such a thing, I can assure you that you will regret it," replied Edward calmly after seeing what was happening to his parents.
"And would that be? Mortal, you are not arrogant enough to think that you could actually hurt me?" asked Death with a sneering tone in his voice.
"Of course not," replied Edward, still with a calm look on his face. "I am not hubristic enough to believe that I could fight a literal God, at least not yet. However, what I can do is to share all my knowledge and understanding of death to all the wizards in this world.
"Hehe, and what would that accomplish," replied Death nonchalantly, still with the sneer in his voice.
"You can pretend all you want, but I know. In the past few years, I have tried many times to call the souls of either my parents or Lily Potter back to the living. However, I never actually succeeded.
"And I know that the reason I failed was not because my magic did not work, but someone actually interfered in the process. I am guessing that it was you who prevented me from succeeding.
"I am also guessing that you secretly investigate what it is that me and Snape are doing. And that after I acquire the Resurrection Stone, then it will be impossible for you to stop me from doing so.
"Now, imagine if all this taboo knowledge were to spread out among all the wizards in the world. Imagine having thousands of wizards trying to revive their dead loved ones, or trying to escape the shackles of death itself. What do you think would happen to the world?"
Death looked deeply at Edward, but it did not say anything. However, Edward did not stop talking:
"I'll tell you what will happen. The cycle of life and death would become chaotic. The fundamental laws of death that govern this world would be affected. And you, the physical manifestation of Death itself, will spend the rest of your lives trying to clean up after these wizards. And if something were to go completely wrong, you might even cease to exist."
"Damn wizards," cursed Death, then with a wave of his hand, Edward was sent out of this white room.
"The arrogance and audacity of this wizard reminded me of that damn old man Merlin. He always thought that he could do whatever he wanted, without paying for the consequences."
After muttering those words, both Death and the soul of Edward's parents also disappeared. However, they were no longer being burned alive.
Both Death and Edward had come to an understanding. Death will treat Edward's parents with the utmost care, and in return, Edward will keep those taboo knowledge to himself.
--Scene Break--
Back to the present, in front of the Mirror of Erised.
"Professor, you are saying that the God of Death is real? And he is in charge of the afterlife?" asked Harry with a look of shock on his face.
"That is correct."
"Are my parents there?"
"Giving that mine are, most likely yours are too, Mr. Potter."
"What is death like then?"
"Well, physically, he looked like a very tall Dementor. As for the experience, Professor Dumbledore once said to me that death was just another adventure. And in many ways, he was correct."
"What is a Dementor?" asked Harry in confusion.
"A very nasty, yet intriguing creature. You will get a chance to meet them later, Potter."
Following this conversation, the two of them became quiet as they stared at the mirror. A few minutes later, Edward spoke again:
"Mr. Potter, if you do not mind. Can you lend me your Invisibility Cloak for a while?"
"No problem, but is there a problem with it?"
"No, there is not. It is just that as an Alchemist, I can feel that your particular cloak is different from the typical ones. And I wished to study it for a while before returning it."
After taking Harry's cloak, Edward thought for a few seconds before taking another Invisible Cloak from his ring that he had placed an Undetected Extension Charm on. This one was made personally by him.
"You can use this one while I am studying yours."
"Professor, you do not have to."
"Potter, think of it as a Christmas present."
Finally, after Edward's persuasion, Harry took the seconds Invisible Cloak and left, while Edward had the Deathly Hallow in his hand.
After reading all the comments, I can see that you guys had no faith in me when it comes to writing intelligent characters. What a shame, what a shame. What...a...shame.