Astaroth was an Abyssal+lord, the demon of prophecy. He is said to have resembled a handsome human with draconic and feathered wings, with a serpentine tongue.
After being rejected by the Queen of Chaos, he infiltrated the legions of Baator and rose to the rank of Treasurer of Hell (at least, so the legend goes), and there he earned the name "Diabolus." He was exposed as a Tanar'ri agent by Gargauth, who drove him back to the Abyss.
For a time, Astaroth ruled a steam-filled Layer filled with floating chunks of burning stone, from which he ruled a cult of mortals on countless worlds, rising to the rank of demigod. Eventually he was slain by Gargauth, who took his divinity and his cult as well.
Astaroth exists now as a Vestige.