The Elder Wand’s power is from it learning from many masters.
Dumbledore says the main reason the Elder Wand has become so powerful is because it’s spent centuries accumulating power and experience from many masters.
“Believers in the Elder Wand, however, hold that because of the way in which it has always passed allegiance between owners — the next master overcoming the first, usually by killing him — the Elder Wand has never been destroyed or buried, but has survived to accumulate wisdom, strength, and power far beyond the ordinary.”
- The Tales of Beedle the Bard
Wands absorb experience from their owners, so a wand that’s passed through the hands of many powerful wizards over the centuries would have absorbed quite a lot of experience.
“As can be seen, Godelot considers his wand to be a helpmeet, almost an instructor. Those who are knowledgeable about wandlore26 will agree that wands do indeed absorb the expertise of those who use them, though this is an unpredictable and imperfect business; one must consider all kinds of additional factors, such as the relationship between the wand and the user, to understand how well it is likely to perform with any particular individual. Nevertheless, a hypothetical wand that had passed through the hands of many Dark wizards would be likely to have, at the very least, a marked affinity for the most dangerous kinds of magic.”
Most wizards prefer a new wand that would shape to their style of magic than a secondhand one, and wands are usually left with their owners after the owner’s death, so most wands remain with one owner and don’t get passed on through centuries like the Elder Wand.
“Most witches and wizards prefer a wand that has “chosen” them to any kind of secondhand wand, precisely because the latter is likely to have learned habits from its previous owner that might not be compatible with the new user’s style of magic. The general practice of burying (or burning) the wand with its owner, once he or she has died, also tends to prevent any individual wand learning from too many masters.”
Though the Elder Wand may or may not have been somewhat more powerful than other wands at the point of its creation, likely the main reason it’s so powerful is because it’s absorbed experience from very many powerful wizards for centuries of use.
Spells cast with the Elder Wand are more effective than normal.
The Elder Wand seems to make spells cast with it more effective. When Harry’s wand breaks, Hermione tries and fails to fix it so that it’s useable again by using Reparo.
“R – Reparo.’
The dangling half of the wand resealed itself. Harry held it up.
‘Lumos!’
The wand sparked feebly, then went out.”
- Harry Potter and the Deathly Hallows, Chapter 17 (Bathilda’s Secret)
Harry asks Ollivander, a skilled and knowledgeable wandmaker, to fix his wand, and Ollivander tells him he doesn’t know of any possible way to fix a wand as damaged as his was.
“Ollivander held out a trembling hand and Harry placed the two barely connected halves into his palm. ‘Holly and phoenix feather,’ said Ollivander in a tremulous voice. ‘Eleven inches. Nice and supple.’
‘Yes,’ said Harry. ‘Can you –?’
‘No,’ whispered Ollivander. ‘I am sorry, very sorry, but a wand that has suffered this degree of damage cannot be repaired by any means that I know of.”
- Harry Potter and the Deathly Hallows, Chapter 24 (The Wandmaker)
Despite that same spell not working when Hermione (who is equally skilled as Harry) tried it, Harry is able to fix his broken wand by casting Reparo on it with the Elder Wand.
“He laid the broken wand upon the Headmaster’s desk, touched it with the very tip of the Elder Wand and said, ‘Reparo.’
As his wand resealed, red sparks flew out of its end. Harry knew that he had succeeded.”
- Harry Potter and the Deathly Hallows, Chapter 36 (The Flaw in the Plan)
The exact same spell that failed to fix his wand before succeeded when cast with the Elder Wand.
The Elder Wand is absolutely not unbeatable in a duel, however.
Though it is a powerful wand, the Elder Wand clearly isn’t unbeatable as its owners have been beaten several times throughout history, as Dumbledore points out.
“What must strike any intelligent witch or wizard on studying the so-called history of the Elder Wand is that every man who claims to have owned it28 has insisted that it is “unbeatable,” when the known facts of its passage through many owners’ hands demonstrate that has it not only been beaten hundreds of times, but that it also attracts trouble as Grumble the Grubby Goat attracted flies.”
- The Tales of Beedle the Bard
In addition to Dumbledore’s defeat of Grindelwald, there’s at least one other known case of a wizard beating the owner of the Elder Wand in a duel - Loxias, who beat Barnabas Deverill.
“All that is certain is that a wand called the “Eldrun27 Wand” by its owner, Barnabas Deverill, appeared in the early eighteenth century, and that Deverill used it to carve himself out a reputation as a fearsome warlock, until his reign of terror was ended by the equally notorious Loxias, who took the wand, rechristened it the “Deathstick,” and used it to lay waste to anyone who displeased him.”
- The Tales of Beedle the Bard
Though the Elder Wand is powerful, being unbeatable in a duel is not one of its powers.