The https://www.wowhead.com/item=40110/haunted-memento is a vanity item that dropped from the invaders during the Scourge Invasion. Whoever has this item in their bags will occasionally give them a https://www.wowhead.com/?spell=53768 buff, which will spawn a https://www.wowhead.com/npc=29238/scourge-haunt that follows the player around for a short while. You used to be able to throw this item to another player if they had enough room in their bags, but this feature was later on removed and the item was made into a Unique item.
Nowadays, this item goes for quite a pretty penny because it’s a vanity collectible item which can no longer be obtained.
However, you can still purchase it from other players on the auction house, and if your realm doesn't have one then you can use The Undermine Journal to search for the item on other realms.