After collecting all the Red Skulls in Specter of Torment, Red will bring Specter Knight to the place of his "death", where he will give him the Donovan Set as well as the Caltrops, saying that they are not magical enough for his tastes and are "hardly worthy of being called a Curio".
This is the only Curio that cannot be upgraded, and the only Curio that may use magic points like Relics. The only other Curio who also act as a relic is the Dread Talon during the rematch Challenge aganist Shield Knight.
If not found in the Memory of Adventure, Luan will personally give Donovan the Caltrops in the beginning of the Memory of Conflict, regarding them as a fine weapon and a possible useful asset for their return trip from the Tower. After initially mocking them as scavenged scrap metal, Donovan in the end comply with his friend and add them in his equipment.