There are almost certainly much older examples, but this was the one I first thought of- slightly older than the stories of Excalibur:
The sword Beowulf uses to kill Grendel's mother.
According to wikipedia:
c. 700–1000 CE (date of poem), c. 975–1010 CE (date of manuscript)
As readable on Project Gutenburg (chapters XXIII - XXIV), Beowulf goes to slay Grendel's mother, but discovers that his (normal) sword was not powerful enough to harm the monster.
The stranger perceived then
The sword will not bite.
The sword would not bite, her life would not injure,
But the falchion failed the folk-prince when straitened:
Erst had it often onsets encountered,
Oft cloven the helmet, the fated one’s armor:
’Twas the first time that ever the excellent jewel
Had failed of its fame.
Beowulf despairs and tries to fight her without his sword, and is protected from her dagger by his armour, and would have died "had God most holy not awarded the victory"
Ecgtheow’s son there
Had fatally journeyed, champion of Geatmen,
In the arms of the ocean, had the armor not given,
Close-woven corslet, comfort and succor,
God arranged for his escape.
And had God most holy not awarded the victory,
All-knowing Lord; easily did heaven’s
Ruler most righteous arrange it with justice;
Uprose he erect ready for battle. (End of Chapter)
He then sees (delivered through divine intervention/deus ex machina?) a giant and magical (?) sword, more powerful than his own (very high quality yet not magical) sword and successfully uses it to penetrate the monster's skin and kill her.
Then he saw mid the war-gems a weapon of victory,
An ancient giant-sword, of edges a-doughty,
Glory of warriors: of weapons ’twas choicest,
Only ’twas larger than any man else was
Able to bear to the battle-encounter,
The good and splendid work of the giants.
Extra, fun note: her blood was so hot and poisonous that it dissolves the blade of the giant sword, and Beowulf can only bring the hilt back.
The brand early melted, burnt was the weapon:
So hot was the blood, the strange-spirit poisonous
The hero swims back to the realms of day.
That in it did perish
It is debatable whether the giant sword is actually "magical" or just giant, but on later inspection later in the book, it is definitely supernatural-not made by normal humans and possessing a greater power than normal humans could have infused it with.
To the age-hoary man then,
The famous sword is presented to Hrothgar.
The gray-haired chieftain, the gold-fashioned sword-hilt,
Old-work of giants, was thereupon given;
Since the fall of the fiends, it fell to the keeping
Of the wielder of Danemen, the wonder-smith’s labor,
And the bad-mooded being abandoned this world then,
Opponent of God, victim of murder,
And also his mother; it went to the keeping
Of the best of the world-kings, where waters encircle,
Who the scot divided in Scylding dominion.
Hrothgar discoursed, the hilt he regarded,
The ancient heirloom where an old-time contention’s
Beginning was graven: the gurgling currents,
The flood slew thereafter the race of the giants,
They had proved themselves daring: that people was loth to
The Lord everlasting, through lash of the billows
The Father gave them final requital.
So in letters of rune on the clasp of the handle
Gleaming and golden, ’twas graven exactly,
Set forth and said, whom that sword had been made for,
Finest of irons, who first it was wrought for,
Wreathed at its handle and gleaming with serpents.