Sthertoth is an evil demon who was formed when the Angel of Light and Shadow split into two halves: A goddess (Mihaele) and a demon. Sthertoth brought darkness to the universe, but was stopped when Mihaele gathered warriors to fight Sthertoth, and sealed him away inside the Celestial Stone and hid it on Planet Kaos.
Unfortunately, two space pirates, Lilith and Rukifellth, found the Stone while exploring Kaos for treasure, and when Rukifellth touched it, Sthertoth possessed him. Under Sthertoth's control, Rukifellth brainwashed all the Elemental Knights to become the evil Astral Knights. They had all the Elemental Stones except the Fire Stone, which Bomberman had with him. One by one, the Astral Knights fell. By the time the Warship Noah became available for docking, only Zoniha and "Bulzeeb" (who was really Bomberman's old rival Regulus) were left. Bomberman and Pommy entered the Warship Noah, where Mihaele took full control of Lilith's body. Soon afterward, Rukifellth learned of Bulzeeb's intentions and took him out with his own Gravity Sphere. After being defeated by Bomberman, Rukifellth revealed that it did not matter whether the Astral Knights beat Bomberman or lost to him — either way, he still gathered the stones in front of the Celestial Stone. He used the opportunity to transform into Sthertoth, the Demon of the Dark.
Bomberman defeated Sthertoth, who then attempted release his full power and use a black sphere to finish off Bomberman, but Mihaele showed up. When Sthertoth said he would destroy her as well, his move was countered by Mihaele's white orb and he was trapped. Mihaele explained to Bomberman that they would reform to destroy the universe and recreate it unless Bomberman was strong enough to defeat them, and Mihaele combined with Sthertoth to form the Angel of Light and Shadow again. Once defeated by Bomberman, the Angel realized that Bomberman would defend the universe. Warning that it would reshape the universe if hope if lost by the next time it awakened, it vanished to lie dormant, releasing Lilith and Rukifellth and bringing all the Astral Knights back to life as their Elemental Knight selves, returning the Elemental Stones.
In an alternate ending, which occurs if Bomberman enters the Warship Noah without the Light and Shadow Stones, or if he is defeated by Lilith's Bludgeoning Blow (which results in Mihaele taking the Elementals' powers for her own), Sthertoth acted more confidently, realizing that Mihaele had the remaining power of the Elementals and anticipating her. Mihaele instead used a black sphere similar to Sthertoth's own to defeat him, but it ultimately had no effect as he gathered the stones to break the seal. He sent Lilith away in darkness, promising to imprison Mihaele in the Celestial Stone once he dealt with Bomberman. He then transformed into his earthly body, the giant and sinister Sthertoth, God of Chaos. Once defeated by Bomberman, he lost his power and disappeared, relinquishing Lilith and Rukifellth. However, he suddenly managed to take over Rukifellth's body with the last of his strength, briefly resuming his true form as he attempted to take everyone down with him in an explosion. The three teleported to the main hangar, but Lilith chose to remain behind because she couldn't bring herself to leave Rukifellth's side, so she went back to that room and closed the portal. Bomberman and Pommy escaped as a bright flash engulfed the screen, presumably taking down the entire Warship Noah and everything still in it.
For all Mihaele's talk about him, Sthertoth is easier than Rukifellth; at first his attacks do not do any damage. Any type of bomb will damage him right after an attack, except for an Ice Bomb, which freezes him (leaving him open for attack, all the same). He can charge at Bomberman, but this attack is easy to avoid, and in the first phase merely stuns Bomberman. If Sthertoth misses Bomberman, he may instead hit a glass wall and break it for an item Bomberman can use. In the second phase, Sthertoth may throw fireballs, but is vulnerable to a kicked bomb while doing do. The third phase is similar, except whenever damaged, he will jump around (causing debris to fall), which is easily avoided if Bomberman keeps moving, and then throw a gigantic fireball at Bomberman, destroying any rubble it connects with. This battle is more an opportunity to heal from the fight with Rukifellth, for the upcoming battle.
In the alternate ending (see above), instead of Mihaele merging with him to form the Angel, Sthertoth transformed into the God of Chaos, Sthertoth. In this battle, Sthertoth's weak point is the jewel on his chest, and he is only vulnerable to Fire and Earth Bombs. He attacks with tornadoes (which can be destroyed with Wind Bombs), laser beams, or the same electricity attack that the Angel uses, though much easier to avoid. However, during the second phase, whenever he takes damage, he will shoot several laser beams at once. Players have discovered that the best way to avoid this attack is to get about 2/3 of the screen away from him and run from the left side to the right side just as he fires, however this isn't always guaranteed to make his attacks miss, especially if Bomberman does not have at least one rollerblade. He can create several tornadoes at once, but they will disappear when he takes damage.
Sthertoth's Japanese name (サートゥス) appears to be derived from Thanatos -- specifically, a transliteration of the Latin form, Thanatus (サナトゥス / タナトゥス), potentially making Thaatus a closer translation; however, 「タナトス」 is a more common transcription of Thanatos. Sthertoth's English name is a mangled variation that obscures this origin (possibly due to Thantos), but it might be intended to be pronounced similarly to Satan.
- Sthertoth's tactics of letting the hero collect all the stones and bring them to him, making the stones his whether or not he defeats his minions, are repeated by Grodus in Paper Mario: The Thousand-Year Door.
- Sthertoth's God of Chaos form briefly appears in the credits sequence of the Good Ending.
- Sthertoth's God of Chaos form bears a striking resemblance to Diaboromon from the Digimon, in shoulder structure and the jewel on his chest.