I missed this the first time I read it, but to me, it seems to pretty clearly refer to creating a dementor - Quirrell doesn't understand what it means because he doesn't know about the true patronus charm.

I think it was implied that he somehow deduced that the dementors are a physical manifestation of death, possibly even before Harry's showcase of the true Patronus spell.

"No," Professor Quirrell said, sounding rather severe. "You don't tell us why, Mr. Potter, you simply tell us that we are not to know. If you wish to devise a hint, you do so carefully, at leisure, not in the midst of conversation."

Harry nodded.

"But," said the Headmaster. "But, but what am I to tell the Ministry? You can't just lose a Dementor!"

"Tell them I ate it," said Professor Quirrell, causing Harry to choke on the soda he had unthinkingly raised to his lips. "I don't mind. Shall we head on back, Mr. Potter?"

"I ate it". Eat death. Death eater.

Quirrell can't perform the true patronus because he isn't as hopeful and positive about the nature of humanity and the vanquishing of death. As dumbledore put it, he doesn't live, but cowers of fear from death.

And then, more interestingly, in chapter 53, when giving Bellatrix the death eater password:

"Those who do not fear the darkness..." murmured Bellatrix.

The snake hissed, "Will be conssumed by it."

"Will be consumed by it," whispered the chill voice. Harry didn't particularly want to think about how Professor Quirrell had gotten that password. His brain, which thought about it anyway, suggested that it had probably involved a Death Eater, a quiet isolated place, and some lead-pipe Legilimency.

Compare it to the plan Harry's dark side came up with on Chapter 81:

Say that, to set up the if-then expectation, and wait for people to understand and laugh. Then speak the fatal truth; and when the Aurors' Patronuses winked out to prove the point, either people's anticipations of the mindless void, or Harry's threat of its destruction, would make the Dementor obey. Those who had sought to compromise with the darkness would be consumed by it.

That's way too nice of a parallelism in prose for it to be a coincidence.

The idea that you can "accidentally" perform a complex secret Dark ritual is ridiculous. Especially because accidental magic has previously always matched the goals of the caster, but Voldemort didn't want Harry to live and certainly didn't want to make him into a living Horcrux (although that would have been cool if he actually did, and had planned that). Also, this seems to be incompatible with how HPMOR magic has already been established to work so EY will need to fix this one.

Eliezer has deviated from canon in such a way that it allows the method for casting dark rituals to be congruent with what happened on Godric's Hollow. We are still missing a lot of information on what Voldemort's intention really was, and if he did intend to kill Harry at all, but there has been speculation on previous threads that Voldemort, in his hubris, accidentally fulfilled the requirements for a certain ritual.

From Chapter 74:

Professor Quirrell's lips twisted further. "Ah, but the truly amusing thing was this. You see, Mr. Potter, the chant of every ritual names that which is to be sacrificed, and that which is to be gained.

And previously on Chapter 43:

"I give you this rare chance to flee," said the shrill voice. "But I will not trouble myself to subdue you, and your death here will not save your child. Step aside, foolish woman, if you have any sense in you at all!"

"Not Harry, please no, take me, kill me instead!"

The empty thing that was Harry wondered if Lily Potter seriously imagined that Lord Voldemort would say yes, kill her, and then depart leaving her son unharmed.

"Very well," said the voice of death, now sounding coldly amused, "I accept the bargain. Yourself to die, and the child to live. Now drop your wand so that I can murder you."

So in this interpretation, Lily Potter's life was to be sacrificed, and Harry's to be gained. What possible side effect would happen when/if Voldemort tried to murder Harry, we can only speculate.

For it is a sad rule that whenever you are most in need of your art as a rationalist, that is when you are most likely to forget it.

Well, the theory of this identity being Riddle has been jossed by Eliezer.

And you're right, I was thinking in terms of Riddle manipulating this other person into being Voldemort's nemesis since the start of his Hogwarts education (given how apparently magically talented this person was) but now after reading what you wrote I realize there is no information regarding this person's magical talent or political alignment while he was attending Hogwarts.

"Born 1927, entered Hogwarts in 1938, sorted into Slytherin, graduated 1945. Went on a graduation tour abroad and disappeared while visiting Albania. Presumed dead until 1970, when he returned to magical Britain just as suddenly, without any explanation for the missing twenty-five years. He remained estranged from his family and friends, living in isolation. In 1971, while visiting Diagon Alley, he fended off an attempt by Bellatrix Black to kidnap the daughter of the Minister of Magic, and used the Killing Curse to slay two of the three Death Eaters accompanying her. Beyond this all Britain knows the story; need I continue it?"

So Riddle/Voldemort murders this person when he is travelling after graduation. He assumes his identity in 1970 and gives this person a solitary life so he can manage his time better better between identities, and also to avoid commiting mistakes that would give away his impersonation in front of his family. A year later he jump-starts Britain's wizarding war setting this other identity as a prominent player in the anti-voldemort side in a single move. Beautiful.

I think most of the confusion surrounding this new character stemmed from the assumption that given his importance in the war, he would have an analogous in canon, and Tom Riddle seemed to fit pretty good (or maybe he is a modified canon character, but I can't really think of any).

Now I'm curious about the details of this impersonation. Amelia Bones mentioned that there was no explanation for his absence, so why did they just assume it was him? Surely someone would have tried a Polyfluis Reverso as soon as they saw him, so it's possible there are darker magics involved, which is totally justifiable with Voldemort. But perhaps it's not that uncommon for wizards seeking power to disappear for a few years and then reappear with a better mastery of magic and a more jaded personality. After all, travelling to exotic places is normal for wizards in canon too.

A problem with Quirrell's heroic alter ego being Tom Riddle, even if the only ones who know Tom Riddle is Voldemort are the inner circle of the Order of the Phoenix, is, why wouldn't madam Bones tell Dumbledore that Tom Riddle is currently inhabiting Quirrell's body?

Quirrell said that he "reported to the headmaster" by which I interpret Dumbledore wants to know in what ways the defense professor was involved in this whole debacle, from which we can infer that he will have or has had Bones report to him as well. The fact that a war hero that was an incredibly powerful wizard as well and led forces against death eaters in various occasions is inhabiting the body of your defense proffesor is not trivial. And then Quirrell also outright TELLS Bones that Dumbledore doesn't know his identity, so we can be sure that Amelia will be reporting that this alleged hero, whatever his identity is, is currently posing as professor Quirrell.

I had assumed before that Dumbledore knew Quirrell's identity, that perhaps he was told that upon his travels after his graduation, Quirinus Quirrell had stumbled upon an ancient tomb containing the soul of a powerful and clever dark wizard, and this spirit had taken possession of Quirrell's body. Dumbledore, accepting this, agrees to let him teach at Hogwarts given that he will be able to teach things to his students few other wizards will be able to. Or something along those lines.

But not before a warning the staff about his nature and making sure things don't get too freaky. From chapter 15:

"Now repeat after me," said Professor McGonagall. (...)

"Even if the current Defense Professor at Hogwarts tells me that a Transfiguration is safe, and even if I see the Defense Professor do it and nothing bad seems to happen, I will not try it myself."

After all, they've had all sorts of wacko defense professors already. From chapter 70:

"My goodness," said Penelope Clearwater. "I think that's the most overtly evil Defense Professor we've ever had."

Professor McGonagall coughed warningly, and the Head Boy said, "You weren't around for Professor Barney," which made several people twitch.

So it's just another year at defense class.

But now it turns out Dumbledore doesn't know his identity? Or does he actually know it and it turns this fabricated heroic identity is not Tom Riddle? If he actually doesn't know, what is he playing at, hiring some incredibly powerful wizard from Merlin knows where who is obviously untrustworthy and likes to plot an awful lot? Dumbledore at one point states the three most powerful wizards in the school are him, Snape, and Professor Quirrell.

And the worst thing is that it actually does fit if the slytherin hero who rose against the death eaters and disappeared suddenly happened to be Riddle.

-Born in 1926

-Sorted into Slytherin

-People speaking of him as the next Dumbledore

-Likes to wander off in Albania

-Of noble house descent

I'm assuming that given how the politics and nobility work in MoR, House Gaunt didn't lose all their gold, or at least not in the same fashion as in canon given that there is probably some sort of convenient system to prevent this. But then again, House Potter did just go broke. And we are told that his grandmother is the Lady of his House. We do never see Merope's mother in canon; so I'm assuming that if the differences from canon are enough to keep House Gaunt from poverty, perhaps it's not all that strange if her mother is alive as well. However, it was Riddle's grandfather, not grandmather, who bore the name of Gaunt. If he died, would her wife inherit his title? I'm thinking not. Or perhaps yes if she was of noble ancestry herself, which is a given considering we are talking about someone who married a Gaunt. And if this new heroic figure wasn't Tom Riddle, then can we assume it was an actual heroic person and Voldemort it taking advantage of his story to cover his usage of Quirrell's body? After all, they were born in the same year, and I can't imagine Riddle manipulating another person since he was eleven years old into being Voldemort's nemesis for a huge social experiment in years to come. Or could he?

I notice I am terribly confused.

And Quirrell does speak about playing the two sides in different occasions, and he did sound to me like he was talking about a social experiment when talking to Hermione, on which he was a leader in both sides of the conflict, and whose outcome he did not expect:

Professor Quirrell shook his head as though in bemusement. "And it was the strangest thing - the Dark Wizard, that man's dread nemesis - why, those who served him leapt eagerly to their tasks. The Dark Wizard grew crueler toward his followers, and they followed him all the more. Men fought for the chance to serve him, even as those whose lives depended on that other man made free to render his life difficult... I could not understand it, Miss Granger."

As soon as I read that Draco was to be pulled out of Hogwarts, I imagined Lucius was going to obliviate or memory charm Harry's influence on Draco away.

It's pretty much a given that Draco was interrogated under Veritaserum by Lucius privately, what we don't know is if Lucius is aware of the total extent of Harry's influence over his son. Precisely what questions would he ask Draco, and based on these questions and the quantity of veritaserum taken, what would the answers be?

He can't know the full extent of what Harry taught Draco, because it would take a lot of time for Draco to explain it, and because Lucius would then also have to forcibly see the flawed logic behind blood purism if he happens to analize what Harry said and it makes sense to him, as he would want to understand the details behind Draco's brainwash (from his point of view) in a deep level. Perhaps. Maybe.

How much would he be willing to invade his son's privacy and forcibly extract information from him? Does he know the gist of what Harry did or in full detail according to Draco's recollections? To Lucius, Draco's knowledge about Harry is extremely valuable intel, and it'd make sense for him to want to know Harry's modus operandi in as much detail as possible.

Draco compared his awakening as a scientist to unknowingly participating in a ritual that irreversibly sacrificed his belief in blood purism. Would Lucius see the irreversibility of this state of mind and decide that the best thing for his son would be to not have this knowledge any more? I assume the obliviation of such a large amount of memories would take its toll on the mind of the mind-wiped as it has happened in canon, but it's more likely Lucius would want to do this carefully and not screw around with his only Son's brain. Would it result in a slightly different personality from the original Draco none the less?