Samael is a Dragon/Fallen Angel hybrid known as the Dragon Eater. As his title implies, he has the ability to destroy other Dragons.
Samael has the upper body of an Fallen Angel (albeit with fangs in his mouth) and the lower body of an Asian Dragon. When he was first summoned by the Hero Faction, Samael was crucified on a large cross with nails driven into various part of his body (arms, tail, wings, etc.) as well as other restraints that had creepy letters written on them. His eyes were also covered with a blindfold that had tears of blood dripping from under it.
Due to the multiple layers of sealing placed on Samael, he is unable to show his personality but according to Ddraig, he has a strong evil intent on Dragons. Issei also mentioned that he sensed various types of negative emotion such as pain, jealousy, suffering, and resentment from Samael.
It is said that long ago, Samael was the perpetrator that influenced the first humans, Adam and Eve, to eat the Fruit of Wisdom, and was cursed by God to become a Dragon because of God's hatred on Dragons and snakes. Following the curse, it was sealed in the deepest part of the Underworld, Cocytus.
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.