On your return from the Markets, several members of your party were beset by a curse. As they wasted away, you frantically searched for the cause and finally found it in Chult. The ruined city of Omu, long lost to history, had become a haven for evil. You were unable to identify the person or persons responsible, but you found the cause of the curse. A necromantic artifact, called the Soulmonger had been activated and, with the help of several wizard towers, you were able to pinpoint its location and teleport directly to it.
Instead of confronting the abomination, you were shunted into an oubliette. Whoever created it had extremely powerful magics and you found yourselves trapped. Waves of necromantic energies eventually wore you down and you succumbed to the power of the Soulmonger. Your souls were ripped from your bodies and sucked into it.
You awake to a fractured sky. Lightning, seemingly frozen in time, snakes across the sky. Beyond it, hundreds of feet above you, the onyx passageways of a labyrinth wrap from horizon to horizon, as if you are inside a large cylinder. Onyx walls, fifteen feet high, rise from a barren floor. An opening leads to a hallway ahead of you.