This command will spawn Addan Deith in Witcher 3.
Can be found on the remains of Mourntart during Contract: The Merry Widow, or purchased from the Scoia'tael merchant in Oxenfurt forest, after he is rescued.
It also appears fairly often as random loot, and one is almost always found in one of the many treasure chests inside the Ancient Crypt, where Geralt goes with Philippa Eilhart during The Sunstone.