How to get the item? Get teleported to the lower part of the Liyue capital and look for the snack vendor “Chen the Swift Blade” there. Talk to him and he will give you the dark iron sword as a reward. This is only available once in the game, so as a collector, be careful not to accidentally use it as a level-up material and activate the lock.
What does the item do? The sword increases your Elemental Lore score and after all reactions from Electro, your attack is increased by 20% for 12 seconds. It might be worthwhile on characters that benefit from Elemental Lore, trigger such reactions, and for which you don’t have a more suitable sword on hand yet. Kazuha, for example, would be a possibility.