According to the writings in The Lesser Key of Solomon, Astaroth is the twenty-ninth spirit listed in the Ars Goetia. He is an exceptionally powerful Duke of Hell assisted by four demons: Amon, Barbatos, Rashaverak and Pruslas. He is depicted as a naked man with a dragon's hands, feet and wings, a secondary pair of angel's wings, wearing a crown, holding a serpent and riding a wolf. He can seduce men through laziness and vanity, make men wise in mathematical sciences and handicrafts, reveal hidden treasures, make men invisible and give them power over serpents. He is sometimes referred to as the prince of accusers and inquisitors, with powers to rival that of the great demons Bael and Beelzebub. His name is derived from the Canaanaite goddess Ashtoreth (or Astarte in Greek).
Astaroth rules over the Forest of Confusion and partially froze it. When the protagonist enters his castle, Astaroth kidnaps the Partner. Once he's defeated, she returns to the protagonist, and the protagonist obtains the Orb of Fear, which grants access to the next area of the Expanse, the Mountain of Horror.
"Origin: Babylonia. The Duke of Fear, one of the 4 Dukes of Hell. It is said he is the form of Ishtar, the Babylonian goddess, in the demon world. She was the goddess of fertility, and the wife of Baal. Astaroth is often depicted as a angel of great beauty, though he smells unbearable. He is a hedonist and can lead people into idleness. He also enjoys sadistic murder."
—Shin Megami Tensei A-Mode DDS dictionary
Along with Surt and Arioch, Astaroth is one of the three major sub-bosses of the last arc of the game and one of the most powerful demons under Lucifer's orders that has taken control over the inferior part of the Basilica.
He appears defending the third floor basement, saying that "there's nothing more pitiful than the man who discards his individuality to willingly become a slave" (referring to the protagonist) before engaging the player in battle if he's not Chaos aligned. This fight, however, is entirely optional, as the player can choose to just take the stairs towards the next floor without having to enter Astaroth's room. After his defeat, he expresses his desire to regain the form of the goddess Ishtar.
In the Chaos route, after locating the throne room of Asura in the last floor of the basement, the hero will need to go back to Astaroth's room, where he will give him the "Wings of Astaroth"—a valuable item needed so Asura can forge the "Devil Ring" and the fight against Michael and his followers will become available.
Astaroth disguises himself as Lucifer's avatar, Louis Cyphre, and attacks Aleph at Briatic Passage. When defeated, he realizes Aleph's power as the Messiah and asks Aleph to return him to the form of Ishtar. This allows the player to fuse Astaroth. When Aleph brings Astaroth to the throne room in Binah, he is separated into Ishtar and Ashtar. Ishtar explains that YHVH forced her to fuse with Ashtar and become Astaroth long ago.
Astaroth is a level 55 Tyrant demon that only appears in New Game Plus. While exploring the maze, a lost soul tells the hero that a demon named Ishtar was locked into a magic coffin and that she can help him in the quest. After locating the Magic Box that lets him face Sin, he will find the coffin, and Astaroth will attack the hero, saying that if she breaks free he will never return back to normal. After his defeat, he tells the hero that "Lucifer" is only playing with him before dying.
Astaroth appears as the final boss of Suginami's Diaspora instance. He spawns in a fixed location on the map while smaller versions continuously spawn by the pillars in the area. The defense of the main form is directly related to how many of the smaller forms have been defeated. He also has a powerful map-wide lightning attack that will disable anyone not in a guard stance. Astaroth plays a role in the Ueno Mirage instance, where he will appear as a boss after Ishtar] is defeated in some variations of the boss room. His presence as a summoned demon by at least one player in a party is required to spawn Ishtar in her true form. He is also the test-giver for Doukan during the later acts for players on the chaos route and defeating him grants players the Devil Ring necessary to access the final fight against Seth in Act 20.
Obtaining the ability to fuse Astaroth requires gathering Masakadus from the end chests of Diaspora and delivering them to Gomory. He is a special triple fusion of Kali, Artemis and Throne. As a reflection of his true nature, Astaroth was one of the few player demons in game with no assigned gender prior to an August 2014 patch.
Astaroth is the Overlord whose summoning is sought by a former Cabinet Minister and Minako in the New Game Plus Challenge Quest, Prevent a Deal with an Overlord. Once the summoning succeeds he laughs, mentioning how it's been some time since he's been in the human world. He plans on looking around after punishing Flynn for trying to stop his resurrection. He retreats once Minako appears and warns that he's still unstable from the travel between both worlds. Once found by Flynn he warns that chasing down a demon is not a wise choice for a warrior, but admires that he wants to finish the job.
Astaroth is in shock as he is defeated and questions what Flynn is before crumbling. Minako reveals that Astaroth was to be the Ring's new king, but thanks him since she obtained his soul regardless of the outcome. His soul is used by her to help track down Mother Harlot and Asherah in the quest Ishtar, Goddess of Harvest.
Astaroth first appears after Raidou defeats Inugami at Dark Tsukudo-cho, mentioning he would like to challenge them to a battle in the future. In the Law and Neutral alignment, Astaroth challenges Raidou to a battle near the end of the game, while he'll simply join them if their alignment is Chaos. If Raidou's alignment is Neutral or Chaos, he will be able to fuse Astaroth after his second appearance.
"One of the 72 demons of the Goetia. A beautiful fallen angel said to be a duke of Hell He appears in the form of a prince riding a dragon and leads an army of 40 evil spirits. He can also travel trough time. Originally he was the goddess Astarte in Phoenicia, but became warped into his current form by the Judeo-Christian God. In Christianity, he is a fallen Throne that lures men into leading indolent lives."
In Catherine, Astaroth is an avatar of Ishtar that draws men into his nightmare world. Although he never physically appears, he communicates with the player by asking them questions in the confessional between stages.