In The Paths of Destiny, according to a quest phase titled "The Sword", the blade is exceptional: "light, sharp as a razor, and fits the hand neatly". In the same entry, it is said that the blade "has a destiny of its own. Time will tell what that destiny is". Possibly referencing a sword with a similar name that was given to Lancelot by the Lady of the Lake in Arthurian legends.
Can neither be bought nor sold
Lady of the Lake knights Geralt after he fulfills her wishes, and gives him this blade along with the words: "Take this sword as a sign of my favor. Once the blade of a great warrior, it lay in the depths for ages, awaiting your arrival."
Blood and Wine expansion
The secondary quest can only be started if you have completed quests which encompass all five of the chivalric virtues. If you have not completed a quest covering that value, you will be told which chivalric value you are missing.
After Geralt completes the Hermit's tasks and defeats him in a trial of combat, he'll disappear into the lake and reappear as the Lady of the Lake, who will then give Geralt Aerondight.
This is one of the only known pieces of equipment whose strength grows with use, and therefore can remain useful throughout a vast number of levels and possibly even a New Game +. It starts out with no slots but can have up to 3 added to it, therefore allowing Runewords to be set into it. When you kill an enemy when it is fully charged its damage increases by 2.
The maximum limit of this bonus damage increases with every character level. If you have maxed out the allotted bonus damage and then kill an enemy while at 10 charges it will not lose the charge, and will keep its double bonus of 100% increased damage and 100% crit rate until you either take damage, the charge counts down or you gain a level to increase the bonus damage limit. If you can keep the bonus damage boost at or at least close to the maximum allowed for your current level as you level up, this sword's damage will usually surpass that of any silver sword you find or can buy at your current level up to and including Relic quality, even through a New Game +, and that isn't including its damage boosting charges.