Since time immemorial, each king, before his death, has constructed a grand mausoleum filled with deadly traps and fantastic wealth. Naturally, tomb raiders had great fun breaking into these tombs and many fortunes were made with the loot therein. One clever king, considered insane in his time (though for other reasons), wasn't happy with this situation, so he worked with prominent adventurers to create a tomb that was meant to be raided. He declared that any who could survive his tomb and the guardians that dwelt within deserved whatever they could retrieve, and tasked an order of monks to keep the coffers stocked. His successors have followed suit, each building a grander and more deadly tomb than the last.
Today, the teams of adventurers that enter these tombs and return with treasure (or even with their lives) are some of the kingdom's greatest celebrities. Religious orders, wealthy benefactors, and historical preservation societies reset the traps as needed and keep the mausolea stocked with historically appropriate valuables. Most are guarded by undead, constructs, and outsiders (beings that need no food or rest), but wild animals and condemned prisoners are sometimes released into the tombs as well. Mages scry into the tombs and, for a fee, let the public track the progress of their heroes as they venture through these challenges.
Naturally, these dungeons attract all sorts: the desperate, the fame-seekers, those seeking to prove their worth, and psychopaths like the players who just want to kill people and take their stuff. Those without proper training tend to suffer any of a panoply of quick but gruesome deaths: decapitated, burned, drowned, torn in half, teleported straight to hell, etc. Teams with strong financial backing are better-equipped than those without, but the corpses and belongings of those who die are left inside the tombs, so it is possible to wander in with little more than the clothes on one's back and pick up still perfectly usable equipment as one goes.
Edit: Does anyone know any synonyms for "tomb"?