| Category | Marvels |
| Location | Bal-Kriav, various systems |
| Owner | none |
| Built | Dawn Era |
This river was established during the Dawn Era by Bruh Kreniik. He had the foresight to make it a military thoroughfare between Ellubôz and Piranoth's Steps. As the worlds of this system were taking shape, Bruh Kreniik's agents discovered that Piranoth was becoming overzealous with his powers of Creation, building too many realms in rapid succession. Bruh Kreniik recognized this as a troubling development, as he believed that crafting worlds necessitated meticulous planning, order, and discipline.
Bruh Kreniik's forecast proved accurate. During the Abyssal Release (7777 DE - 8777 DE), the primordials were compelled to vacate what had long been designated as Piranoth's Steps. The Abyss's new overlords were its Demon conquerors.
The Styx enters the realm of Barbamor, in the Abyss, through a permanent rift to the world of Avernus. On both sides of this rift stand deep, fortified structures dating back to the Creation War. These defenses have been continuously augmented for millennia, creating seemingly impregnable fortifications that extend for many miles. On the Abyss side, one hundred thousand demons man their walls, while the Avernus side boasts an equally formidable army of devils. The decision-makers on both sides consider the defenses so robust that neither has used the Styx as a route for inter-system assault since the Horgon Era. The shores of the Styx for a hundred miles into Avernus are littered with the remnants of failed naval expeditions, the hulks of which now serve as abodes for fiends and other hellish creatures of Avernus. Despite the many wars and the deaths of millions on both sides, some have managed to infiltrate or bypass the enemy's defenses, often skilled spies or assassins capable of evading the hundreds of sentries along the five-mile-long corridor of walled river on the Abyss side, or the eight-mile-long stretch on the devil's side.
The river Styx traverses numerous realms, with an unquantified number of tributaries. It possesses branches winding through select domains of Pandemonium, the Abyss, Tartarus, Hades, Gehenna, and Acheron. The river is reputed to terminate at an ocean on Stygia, although some speculate the beginning and end of the Styx are one and the same.
One of the Styx's tributaries periodically enters the world Bal-Kriav at a location known as the Styx Fountain. As the waters of the Styx, famed for inducing permanent forgetfulness, mingle with the Underdark waters of Maitimírë, their potency diminishes. The granite cavern walls that border the lake and extend down the river leading to the Green Nebulous are tainted by contact with the Styx. The granite of this region that becomes tainted adopts the properties of Styx Granite.