The City of Strangers. The Beautiful City. The City of Gold. The City of Sharks.
A dangerous, alien plane where glowing frog-slimes slither and slurp, metal-skinned bird people rebuff diplomacy, and only the endless subterranean tunnels are safe (just mind the gravity-bending moles). Visitors will have to skulk around in the dark to avoid the curse carried by the sunlight. Because (have I mentioned?) the sun is the source of a powerful curse--one that covers the whole planet.
Mondaloa is a name shared by both a city and a deity. Mondaloa, the city, is built on layers of crypts and tombs that are far more opulent and majestic than the city that covers them. Mondaloa, the deity, is the god of rest, peace, and death. There is nothing Mondaloa desires more than to see the dead buried deep in their tombs where they can rest in honor and peace. But there is trouble: something is torturing the dead of the city, and driving them to madness and rage. Now, 500 years of honored ancestors are trickling into the city above, seeking blood and pain and death.
The air had grown chill the minute they descended into the strange valley, which was unmarked on any of their maps. It was so strange here, devoid of animal life and completely silent. The horses were nervous the entire journey through the vale. As they set about to collect firewood for the campfire they could hear their own voices as dim echoes through the eerie silence.
The food didn't taste anything that evening and their sleep was cold and troubled by nightmares. While they are clearing camp the next morning, one of them stumbles over a piece of stone jutting out of the ground nearby the horses. On closer inspection there seems to be runes engraved into the polished surface. The symbols true meaning is no more known among mere mortals and if they decide to dig deeper, they will discover that it is an ancient altar buried within the soil.
Any historically oriented party member will recognize the largest symbol to be the insignia of the powerful warlock who ruled this realm several centuries ago. At their departure from the area, something will seem amiss with one of the party members and all will remember the stories of the warlock's thousand curses.