The Infernal Cup is the demonic equivalent to the Mortal Cup, used to turn Shadowhunters into Dark Shadowhunters, changing their angelic alliance to demonic. It was forged by Sister Magdalena and demonized by Sebastian Morgenstern with the blood of the summoned Greater Demon Lilith.
It is a glassine chalice and an almost exact replica of the Mortal Cup, except for its midnight black color. It was originally the color of silvery white adamas before Lilith infused her blood into the Cup.
Sebastian had been planning the creation of the Cup for weeks. He tasked a former member of the Circle and the former Iron Sister, Sister Magdalena, to forge it. Shortly after securing everything necessary to imbue the Cup with its power, he sent Jace to retrieve the cup from Magdalena and kill her immediately after.
With the Cup, Sebastian was able to affect the whole Nephilim society in a drastic way, having Turned a huge number into Endarkened Ones for what would later be dubbed as the Dark War. The Silent Brothers and warlocks tried to find ways to cure the affected Nephilim, to no avail. Nonetheless, the Cup was destroyed by Jace after, upon Sebastian's instruction, throwing it into the demonic summoning circle, effectively killing all the Dark Shadowhunters it created. Despite the destruction of the Cup and the Endarkened Ones, the sudden drop in the number of Shadowhunters was part of what caused the reopening of the Shadowhunter Academy.