Does using Felix Felicis while in Priori Incantum improve your chances of winning the duel by strengthening your spell?

Question

Does using Felix Felicis while in Priori Incantatem improve your chances of winning the duel by strengthening your spell against your opponent's spell?

Answer 1

Its effect would depend on several factors.

The effects of taking Felix Felicis on a Priori Incantatem battle are impossible to predict since several factors would be involved, such as the skill of both the combatants. Though Felix Felicis does give the person who drinks it good luck, and so would improve their chances, it in no way would guarantee a win.

Felix Felicis can increase odds of winning.

Using Felix Felicis is banned in organized competitions, which logically implies that using Felix Felicis increases the odds of being successful in any competitions participated in under its effects.

“Now, I must give you warning that Felix Felicis is a banned substance in organised competitions … sporting events, for instance, examinations or elections. So the winner is to use it on an ordinary day only … and watch how that ordinary day becomes extraordinary!”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 9 (The Half-Blood Prince)}}

Therefore, it can increase odds of winning, which would likely extend to winning in Priori Incantatem as well.

Felix Felicis can help magical ability a bit.

Felix Felicis also does help magical ability somewhat, since Harry is able to cast a Refilling Charm nonverbally while using it when he hadn’t managed to do it before without it.

“The Felix Felicis gave Harry a little nudge at this point and he noticed that the supply of drink that Slughorn had brought was running out fast. Harry had not yet managed to bring off the Refilling Charm without saying the incantation aloud, but the idea that he might not be able to do it tonight was laughable: indeed, Harry grinned to himself as, unnoticed by either Hagrid or Slughorn (now swapping tales of the illegal trade in dragon eggs), he pointed his wand under the table at the emptying bottles and they immediately began to refill.”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 22 (After the Burial)}}

Therefore, it’s possible that it could help win Priori Incantatem, since it does help in using magic well.

But Felix Felicis can’t do the impossible.

However, as Hermione explained to Harry, Felix Felicis only improves luck, and luck can only get you so far on its own. It improves the chances of success at things the person who drinks it was already capable of doing, but it doesn’t make it possible for them to do things far beyond their skill level, and it doesn’t make the impossible suddenly possible, because luck isn’t enough for that.

“I think I’m going to take another swig of Felix,’ said Harry, ‘and have a go at the Room of Requirement again.’

‘That would be a complete waste of potion,’ said Hermione flatly, putting down the copy of Spellman’s Syllabary she had just taken out of her bag. ‘Luck can only get you so far, Harry. The situation with Slughorn was different; you always had the ability to persuade him, you just needed to tweak the circumstances a bit. Luck isn’t enough to get you through a powerful enchantment, though.”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 24 (Sectumsempra)}}

Therefore, it won’t make someone win Priori Incantatem if they’re simply incapable of defeating their opponent.

And Felix Felicis won’t win every battle.

Also, Felix Felicis doesn’t guarantee that in a battle, the person who drinks it will defeat their opponents. When the members of Dumbledore’s Army take it before a confrontation with Death Eaters, it only makes the Death Eaters’ curses all miss them. It doesn’t make them suddenly able to defeat the Death Eaters in battle.

“And a Death Eater’s dead, he got hit by a Killing Curse the huge blond one was firing off everywhere – Harry, if we hadn’t had your Felix potion, I think we’d all have been killed, but everything seemed to just miss us –”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 29 (The Phoenix Lament)}}

Therefore, it also won’t necessarily guarantee a win in Priori Incantatem against a vastly more skilled opponent, it might just keep them from being killed or injured.

Answer 2

The nature of Felix Felicis makes the answer "it depends".

Under the potion's influence, the duel will have whatever outcome is necessary for your overall purpose to become closer to being achieved. If there is a sequence of events where you lose the duel, but end up "winning in the end" because of some chain of coincidences your loss in the duel sets off, Felix Felicis will make you lose the duel. Not all duel losses are fatal.

Answer 3

It's not entirely clear how powerful Felix Felicis is. The clearest statement of its limitations comes from Hermione in Chapter Twenty-Four of Half-Blood Prince:

"I think I'm going to take another swig of Felix," said Harry, "and have a go at the Room of Requirement again."

"That would be a complete waste of potion," said Hermione flatly, putting down the copy of Spellman's Syllabary she had just taken out of her bag. "Luck can only get you so far, Harry. The situation with Slughorn was different; you always had the ability to persuade him, you just needed to tweak the circumstances a bit. Luck isn't enough to get you through a powerful enchantment, though. Don't go wasting the rest of that potion! You'll need all the luck you can get if Dumbledore takes you along with him..." She dropped her voice to a whisper.

According to this, Felix Felicis cannot overpower powerful enchantments, but it can tweak the circumstances a bit.

There are also a couple of descriptions of Felix Felicis in Chapter Nine of Half-Blood Prince:

"It's liquid luck," said Hermione excitedly. "It makes you lucky!"

"Quite right, take another ten points for Gryffindor. Yes, it's a funny little potion, Felix Felicis," said Slughorn. "Desperately tricky to make, and disastrous to get wrong. However, if brewed correctly, as this has been, you will find that all your endeavors tend to succeed ... at least until the effects wear off."

Both of these descriptions are somewhat vague, lacking precise parameters. However, we can perhaps get a better idea of what Felix Felicis does by examining the two known instances where it was used.

When Harry uses it in Chapter Twenty-Two of Half-Blood Prince the first description we get is:

Harry did not answer for a moment. Then, slowly but surely, an exhilarating sense of infinite opportunity stole through him; he felt as though he could have done anything, anything at all... and getting the memory from Slughorn seemed suddenly not only possible, but positively easy....

He got to his feet, smiling, brimming with confidence.

This does not particularly indicate any great luck or successful endeavors. Indeed at this point it seems like nothing more than a confidence-boosting potion.

Over the few hours that Harry was under its influence we find several things that it seems to have done:

• It directed him to visit Hagrid.

  "Excellent," he said. "Really excellent. Right... I'm going down to Hagrid's."

  "What?" said Ron and Hermione together, looking aghast.

  "No, Harry — you've got to go and see Slughorn, remember?" said Hermione.

  "No," said Harry confidently. "I'm going to Hagrid's, I've got a good feeling about going to Hagrid's."

  "You've got a good feeling about burying a giant spider?" asked Ron, looking stunned.

  "Yeah," said Harry, pulling his Invisibility Cloak out of his bag. "I feel like it's the place to be tonight, you know what I mean?"

• It cleared the path through the castle.

  His feeling of elation increasing, Harry strode off through the castle. He did not have to creep along, for he met nobody on his way, but this did not surprise him in the slightest. This evening, he was the luckiest person at Hogwarts.

• It caused the door to be left unlocked.

  When he reached the entrance hall he saw that Filch had forgotten to lock the front door.

• It directed him to a route that would pass Slughorn.

  It was when he reached the bottom step that it occurred to him how very pleasant it would be to pass the vegetable patch on his walk to Hagrid's. It was not strictly on the way, but it seemed clear to Harry that this was a whim on which he should act, so he directed his feet immediately toward the vegetable patch, where he was pleased, but not altogether surprised, to find Professor Slughorn in conversation with Professor Sprout.

• It directed him to reveal himself to Slughorn.

  Seized with an immediate desire to reveal himself, Harry pulled off the cloak with a flourish.

• It directed him to tell the truth.

  "Well, sir, it's Hagrid," said Harry, who knew that the right thing to do just now was to tell the truth.

• It showed him how to attract Slughorn.

  And now Harry saw clearly what was to be done. "Well," he said, with a most convincing hesitancy, "well, if you wanted to come, Professor, Hagrid would probably be really pleased.... Give Aragog a better send-off, you know..."

• It advised him to tell Hagrid about Slughorn.

  He broke into sobs again and Harry resumed the patting of his elbow, saying as he did so (for the potion seemed to indicate that it was the right thing to do), "Professor Slughorn met me coming down here, Hagrid."

• It directed him not to drink.

  Harry, however, with the way ahead illuminated for him by Felix Felicis, knew that he must not drink, so he merely pretended to take a gulp and then set the mug back on the table before him.

• It advised him to refill the drinks, and he was able to do it nonverbally.

  The Felix Felicis gave Harry a little nudge at this point, and he noticed that the supply of drink that Slughorn had brought was running out fast. Harry had not yet managed to bring off the Re-filling Charm without saying the incantation aloud, but the idea that he might not be able to do it tonight was laughable: Indeed, Harry grinned to himself as, unnoticed by either Hagrid or Slughorn (now swapping tales of the illegal trade in dragon eggs) he pointed his wand under the table at the emptying bottles and they immediately began to refill.

• It led him to continue talking about his mother.

  "I forgot," lied Harry, Felix Felicis leading him on. "You liked her, didn't you?"

• It told him that Slughorn wouldn't remember what happened.

  He knew he was safe: Felix was telling him that Slughorn would remember nothing of this in the morning. Looking Slughorn straight in the eye, Harry leaned forward a little.

This can be contrasted with what happened when the Felix Felicis wore off:

Harry could feel the Felix Felicis wearing off as he creeped back into the castle. The front door had remained unlocked for him, but on the third floor he met Peeves and only narrowly avoided detection by diving sideways through one of his shortcuts. By the time he got up to the portrait of the Fat Lady and pulled off his Invisibility Cloak, he was not surprised to find her in a most unhelpful mood.

If we look at all the things that Felix Felicis did, we don't find any major manipulations. It seems to have consisted almost entirely of directing Harry in what to do, apparently based on some sort of knowledge of the near future. In that sense it didn't really manipulate magic in any sense. This would explain why users of the potion would generally be successful – it helps them make the best choices – and at the same time allows us to understand why it wouldn't be useful against powerful enchantments – it doesn't really affect magic.

Even the few things that might seem like actual interferences with the running of the world, cannot necessarily be taken at face value. The fact that the door was unlocked and Harry met no one on the way does not prove that Felix Felicis did anything. This is a subjective interpretation of a sample size of one. It's altogether possible that there was some other reason why those things happened, or Harry simply got lucky. Even Harry's ability to cast the spell nonverbally can be simply attributed to the extra confidence Felix Felicis gave him.

The point is that nothing in this episode gives any indication that Felix Felicis could strengthen a spell, or directly alter a duel. This brings us to the other incident in which it was used. We only get one short description of its effects there in Chapter Twenty-Nine of Half-Blood Prince:

"Neville and Professor Flitwick are both hurt, but Madam Pomfrey says they'll be all right. And a Death Eater's dead, he got hit by a Killing Curse that huge blond one was firing off everywhere – Harry, if we hadn't had your Felix potion, I think we'd all have been killed, but everything seemed to just miss us –"

This attributes the missing curses to Felix Felicis. This does not show any direct effect on magic. This can be simply explained as Felix Felicis directing those who used it so that they wouldn't get hit. I.e. just like Felix Felicis could tell Harry where to go to find Slughorn, it could tell Ron where to go to avoid being hit by a curse. Moreover, this instance suffers from the same limitation as the first one: it is an uncontrolled subjective experience. We can't say with certainty that Felix Felicis actually did anything. All we know is that the people who took it were not hit by curses. This could be explained even without Felix Felicis as well.

Additionally, it clearly did not help them in certain ways. Malfoy was still able to get past them by using Peruvian Instant Darkness Powder and his Hand of Glory. Again, this tells us that Felix Felicis probably does not have much direct effect on magic and/or events.

It would seem from all of the above that the only things we can safely say about Felix Felicis is that it increases confidence, and can lead you in the right direction. Both of those things can give someone an advantage in a competition, which is why it is banned. However, we don't have evidence that it could directly impact the power of a spell.

So, returning to the question here:

Does using Felix Felicis while in Priori Incantum improve your chances of winning the duel by strengthening your spell against your opponent's spell?

The answer might be yes and no. It probably wouldn't directly strengthen the spell, but it would give the user increased confidence and it might direct the user what to do at any given moment in the duel which would tend to give that person an advantage.