Nergal

"You can't save her."

- Nergal to John Constantine before taking Astra Logue to Hell.

Nergal is a powerful demon who was summoned by John Constantine in a botch attempt to defeat another demon possessing Astra Logue, only for Nergal to drag Astra into Hell killing and damning her for all eternity. As a result, Nergal was also able to damn John's soul so it will be dragged into Hell upon his eventual death, which John tries to prevent by atoning for his sin.

Biography
Astra Logue was possessed by a demon, causing John Constantine to conjure up Nergal from Hell intending to compel him to drag the lesser demon back to Hell and save Astra. Instead, Nergal killed Astra violently and dragged her soul to Hell where she was subsequently damned for eternal suffering. This led to John suffering immense guilt and often having nightmares of the event. Ritchie Simpson, who was present for the event, also frequently endured nightmares and blamed John for Nergal's actions.

Nergal sent Furcifer to kill the psychic Liv Aberdine but John was able to exorcise him. Before doing so John told Furficer to tell Nergal that he was coming for him and Astra.

Powers

 * Demons Ruling: As Nergal is most likely a high-ranking demon, he has authority over powerful demons like when he sent Furcifer to kill Liv.
 * Telekinesis: When John had a dream of Nergal, the last make Astra come to him by simply tending his hand at her.
 * Interdimensional travel: Nergal can travel from Hell to Earth.
 * Immortality: As a demon, Nergal is most probably immortal.

Behind-the-scenes

 * In the DC comics Nergal was also a demon responsible for Astra Logue's death and condemnation to Hell.
 * Nergal is also the name of a most respected Mesopotamian deity that represents a certain phase of sunlight. Ironically, the demon Nergal is a villain instead of a heroic god.