Under the Mantle of Madness
The time of chaos preceded the PC’s birth, when a cataclysm caused the elements themselves to take flight and battle each other: mountains ripped from their moorings, seas gathered up as a great lumbering beasts, skies choking with smoke, flares from the sun hurtling like comets.
What few humans survived the initial apocalypse sought refuge in the Underdark. The PC’s were among them. Their group found homes in the lair of Ruholla, an ancient black dragon, amid her massive vault under earth, miles across, and a mile high to the chimney. In exchange for their pitiful lives, subsistence farms, and cattle-herding, the community offers Ruholla and her seven princes and princesses regular sacrifices.
When the time comes to offer the PC’s in sacrifice to Ruholla, the PC’s families demure. During her foraging for food, one of the parents has seen a small safe place beyond the Vault, while another has discovered an ancient scroll that can teleport the PC’s there safely . They prepare the PC’s with what limited provisions they can offer, and bid them to end the tyranny of Ruholla, and to find a way for them to return to the surface world in safety.