The Plunderers daggers weapon in Final Fantasy XV can be upgraded for better stats by talking to Cid who gives players the A Better Pair of Plunderers quest. The quest will require Noctis to bring Cid a treasure item by the name of Barbed Scythe so the weapon can be upgraded.
As of right now, there seems to be only one place in the game where players can find them and that is the Malmalam Thicket dungeon. The dungeon is actually not like other dungeons and is more of a thick forest area between mountains which means it has some unique monsters instead of the usual Imps the players find in most dungeons.
The access to Malmalam Thicket dungeon is located to the southwest of the game map, to the east of the giant volcano. The actual Barbed Scythe item is dropped by killing Soldier and Killer Wasps in the area. The drop rate isn’t guaranteed so players will need to progress further into the dungeon to encounter more wasps until they finally find the item.
The area has enemies around level 37 and many of them cause Confusion so it is best to take countermeasures for that before venturing in. Once players find a Barbed Scythe from one of those wasps, they can either proceed to completing the dungeon for the Royal Arms at the end (which happens to be a really awesome scepter) or simply return to the entrance.
Having acquired the Barbed Scythe, Noctis can return to Cid and hand over the item so the upgrade can begin. Once Cid starts working on it, players can go and rest somewhere and Cid should be done in that time.