How would Slughorn have known how to cast the Dark Mark?

Question

In Half Blood Prince, once Dumbledore has determined that no one had been harmed or killed and has found Horace Slughorn transfigured into an armchair, Dumbledore mentions that the Death Eaters would have left the Dark Mark had they been at the house, to which Horace responds:

"I knew I was forgetting something! Wouldn't have had time to conjure it anyway."

But in Goblet of Fire, Mr. Weasley says that only Death Eaters know how to conjure the Dark Mark. How would Horace manage this feat unless he was a Death Eater? Or was this just a flippant comment that Horace makes?

Answer 1

Actually, the curse is known by others.

The incantation is Morsmordre. In the 4th book, Harry and Hermione overhear Barty Crouch Jr. cast the spell while he is using Harry's wand. When the ministry shows up, Cedric's dad accuses Winky of casting the spell, at which Barty Crouch Sr. says something to the effect of "Do you think I go around teaching my servants the dark mark?". This clearly shows that Barty Crouch Sr. knows or could be expected to know the spell, as well as probably the rest of the Ministry of Magic staff standing in that clearing.

In book 6, after Dumbledore's death, the teachers are easily able to dispel the mark as well, which shows they have some familiarity with it, the ability to read into the magic enough to know how to dispel it. A wizard of higher caliber should be able to mimic a simple spell like the dark mark anyway, as the kids are able to make "firework" type flares easily, making a symbol in the sky shouldn't be much harder.

Answer 2

I took that as a statement of the level of knowledge possessed by Horace. The fact that only Death Eaters know the spells, doesn't mean that only they can conjure it, it means it's knowledge that is transferred inside their circle. I'm betting skilful magic wielders, like Dumbledore, Horace and others know the spell and what it takes to use it.

Remember that after Voldemort's fall, many Death Eaters went into the light side, and they most likely revealed some trade secrets.

Answer 3

There is always the possibility that Arthur Weasley is wrong, or making a "lie told to children" oversimplification. If Dumbledore can be wrong from time to time, why not Mr. Weasley?

Answer 4

Slughorn most likely didn’t know the Dark Mark’s incantation.

The correct incantation to conjure the Dark Mark was something that most likely only Death Eaters know, which is what Arthur Weasley kept trying to remind Amos Diggory. Crouch was blustering and acted quite offended at the suggestion that Winky cast the Dark Mark - but he would have suspected it was his son he’d smuggled in, and be trying to throw suspicion off himself.

“You’ve been caught red-handed, elf!’ Mr Diggory roared. ‘Caught with the guilty wand in your hand!’

‘Amos,’ said Mr Weasley loudly, ‘think about it … precious few wizards know how to do that spell … where would she have learnt it?’

‘Perhaps Amos is suggesting,’ said Mr Crouch, cold anger in every syllable, ‘that I routinely teach my servants to conjure the Dark Mark?”
_{^{- Harry Potter and the Goblet of Fire, Chapter 11 (Aboard the Hogwarts Express)}}

Arthur Weasley, who also worked for the Ministry and had no ulterior motivation, said it’d be highly unlikely that the person who’d conjured it would have never been a Death Eater.

“But I’ll tell you this … it was only the Death Eaters who ever knew how to conjure it. I’d be very surprised if the person who did it hadn’t been a Death Eater once, even if they’re not now … Listen, it’s very late, and if your mother hears what’s happened she’ll be worried sick. We’ll get a few more hours’ sleep and then try and get an early Portkey out of here.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 11 (Aboard the Hogwarts Express)}}

He was proven right about this when it turned out to be Barty Crouch Jr. who’d conjured the Dark Mark - he was indeed a Death Eater. So, it seems highly unlikely that Slughorn, who wasn’t one and kept away from war, would have known the right incantation to cast the Dark Mark.

He could, however, have attempted to make a fake one.

While only Death Eaters knew the correct incantation to conjure the Dark Mark, other wizards knew what it looked like, so Slughorn could have attempted to “forge” it. He’d used dragon blood to fake human blood, so he could have tried something similar. Fred and George were able to make candy Dark Marks, presumably without knowledge of how the real Dark Mark was cast.

“Come through to the back, Harry, that’s where we’re making the real money – pocket anything, you, and you’ll pay in more than Galleons!’ he added warningly to a small boy who hastily whipped his hand out of the tub labelled: Edible Dark Marks – They’ll Make Anyone Sick!”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 6 (Draco’s Detour)}}

Should he have thought of it, Slughorn might have tried replicating the Dark Mark - it might not be indistinguishable from the real thing, but he’d likely have tried to make something like it.