I thought how to answer that open ended question, which could open an unwanted number of can of worms, and decided to throw caution to the wind.
"Will you swear to not reveal certain elements of my answer to anyone, without my explicit permission to do so? Only then, can I answer that question. And I can even explain the exact circumstances and my knowledge about this effect in great detail."
He looked shocked at my question, but nodded nevertheless, as he took out his wand, and swore to it. "So more it be." So saying he lit his wand, wandlessly.
"Now I can either start with what happens when the said effect occurs, or I could tell you how it occured, or I could tell you how I know of it. But you are explicitly forbidden from revealing the knowledge of where it actually happened, how I came to know of it, and anything about the two participants of this effect. Is that clear?"
He smiled and nodded, and I regaled the story of Harry Potter, his life and times until his conflict with Voldemort, and his eventual duel in Little Hangleton.
He looked shocked and said, "How come do you know of this? I would have rejected this story as lies, were I not aware of the said wand made of Holly, resting in my own shop."
I smiled and said, "Because Fate showed me the answers, of what would happen if I was not born, and didn't endeavour to end Dark Lord's tyranny, like I did."
He looked flabbergasted, as his jaws hung low and his eyes bulged at my unexpected bomb. "You are the Vengeful One?"
I smiled and nodded. "And you are forbidden from ever telling another soul about it, as it is also part of the story, of how I came to know of the effect."
He shook his head, and smiled at me. "You have nothing to fear from me, sir. I will carry this secret to my grave, if you so wish. But it seems even more improbable, than the tale of Harry Potter."
I smiled as I had the perfect evidence of my crime. I took out three wands, and brought them to the table, where we both sat. "I have heard, how you recognise every single wand you have sold in your life. Could you tell me, who they belonged to?"
He looked down, and watched, with each passing moment, he looked surprised and shocked all over. "Bellatrix Lestrange, Bartemius Crouch Junior and Walden Macnair."
I smiled at him. "I rescued a few muggleborn who were imprisoned in the Malfoy Manor, when I visited the place. I spared Lucius, because he was important in bringing down Tom Riddle, but from what I think, you already knew. The first time you saw his wand, I mean.
Seeing his sad and guilty nod, I continued, "Anyway, the muggleborn were going to be a chore to free, and they would have been helpless without a wand, so I decided to give them the wands I kept as trophy, from all the people I killed. I thought it would have been a delicious but of irony, the wands of Muggle haters used by muggleborn wizards to escape their inevitable death and suffering."
I laughed at that. I held a short wand, and said, "But Barty's wand was sentimental to me, Bellatrix's wand would have worked for me, when I was hunting them, as I could sense the Hebridean Black core from it, and Walden Macnair was the last of the Pureblood Death Eaters who was on my target. So you see, I have, just like Voldemort, done great and incredible things, terrible no doubt, but great, nonetheless."
We both sat silently at that, neither wanting to speak anymore about it. But Garrick, after a while, stood from his place and shook my hand.
"No, Mr. Aethelred, I believe you are wrong. If things would have come to pass as you saw them, then it is truly better that you did what thought was right. If more innocents were saved, and the many deaths were prevented if you had to kill a few wizards who were killers themselves, then you have no shame and dishonour to bear, as you did what no one would have the courage and will to do in your place. And even if no-one knows of it, I will forever be grateful for you sacrificing your innocence, and doing what adults should have done, instead of needing a child like you to stain your hands for our freedom."