Cocytus is a location in Persona 3 FES. It is the second door in the Abyss of Time. It appears the same as the Arqa Block of Tartarus, with the faces replaced with purple oval gemstones and the blood into splashes of purple liquid. After the 9th floor, it takes on the appearance of a worn down and corroded Tziah Block with eroded bricks in place of the red carpets. On the 15th floor, there is a door into Ken Amada's past.
Cocytus is one of the rivers of Hades in classical Greek mythology - the "river of wailing"; it is also known as one of Hell's water bodies in Dante Alighieri's Divine Comedy - the frozen lake at the Ninth Circle, where traitors are buried in the lake's ice, from neck-high to completely being encased in the lake's icy waters. Satan is at the Ninth Circle's deepest reaches, the futile flapping of his wings only increasing the violent ice storms of the area and further chaining him to the ice of the lake.