Samael is a Dragon/Fallen Angel hybrid known as the Dragon Eater. As his title implies, he has the ability to destroy other Dragons.
It is said that Hades summoned Samael from the Underworld and temporarily gave it to Cao Cao as part of the alliance. During the battle between Cao Cao and the Gremory and Vali teams, Samael's poison was injected into Vali's body, while Issei received the poison through an arrow fused with Samael's blood (given to him by Hades) during his fight with Shalba Beelzebub. He is later resealed back into the Realm of the Dead.
Powers & Abilities
Samael has a powerful curse that is extremely deadly to both Dragons and Dragon possessors alike. The poison can also affect snakes or snake-related beings. Even Ophis and Great Red are not immune to this effect. According to Azazel, Samael is the "Evil" from the God of the Bible made from His hatred and its existence alone can not only extinguish Dragons, but it can also affect other beings as well, resulting in it being sealed within the depths of Cocytus.
The curse even resides within Samael's blood, allowing others besides Samael to use the curse (usually by infusing it into a weapon).
Samael's role is similar to Satan; being the serpent that tempts Adam and Eve to eat the forbidden fruit and becoming a Dragon after his fall.