Legendary Weapons in Diablo 4 are some of the most powerful items in Diablo 4, capable of providing players with incredible bonuses and abilities that can help them overcome even the most challenging foes. These weapons are rare and difficult to obtain, but their power makes them well worth the effort. Players can obtain legendary weapons in a variety of ways, including as drops from powerful enemies, rewards for completing challenging quests, or by crafting them using rare materials and recipes.
Legendary Weapons become "Legendary" by having a Legendary Aspect imprinted into them. This means that the weapon "per se" isn't the legendary one, unlike previous Diablo entries, in which you always encounter the same fixed Legendary Weapon, with a unique name. To learn about those kinds of weapons in Diablo 4, please head to Unique Weapons.
All Legendary Weapons can only have one Legendary Aspect at a time. Adding a second Legendary Aspect, will always overwrite the previous one, without exception.