What are legendary weapons in lol?
From the LoL wiki: "A legendary item (or high tier item) is an item that is crafted from at least one advanced item and a recipe. It may or may not contain basic items."
Basically the best way to think about a Legendary item is they are recipes inside recipes. For example: Shurelya's Revererie requires Kindle Gem among other things. The Kindle Gem also has a recipe which requires a Ruby Crystal. So essentially the Kindle Gem has a recipe then Shurelya's Revererie also has a recipe so that makes it a legendary item.
They are the items or weapons that you usually build last and are composed of other lower tier weapons. They are usually priced as the more expensive weapons and can be considered items that dictate the game. Bloodthister, A life Steal and + Attack Damage item, is a legendary item/weapon. These weapons usually have better effects/grouped effects from the items its made from.