First Blood (First Dragon Blood in the Japanese version) is a consumable DLC item in Fire Emblem Fates. It allows a unit that consumes it to utilize Dragon Veins.
First Blood is a one-time consumable item that permanently gives any unique unit that consumes it the ability to utilize the Dragon Vein ability. If used by a first generation unit, their children will also inherit the ability so long as they are recruited after the item has been used on their parent(s). Logbook units are unable to use this item, most likely for balancing reasons since they can be bought again from the logbook if they fell in battle on Classic mode.
It is obtained every time the Hidden Truths 2 DLC chapter is completed.
The ability to access Dragon Veins stems from possessing the bloodline of the ancient First Dragons, who once empowered humans with drops of their own blood. In Hidden Truths 2, Anankos provided his blood to Odin, Selena, and Laslow to allow them to temporarily use Dragon Veins to fight off an ambush from Vallite forces while they were in Valla.