Aliens are going to strip mine earth, but to give the illusion of fairness those who survive the initial collection are given a choice, live on the surface knocked effectively back to the stone age or enter into an 18 floor dungeon that is effectively the galaxies largest game show.
The game is serious, the deaths are real and grusome. In the interest of fairness theres an AI controlling the game and the crawlers can change race, choose classes, level and gather loot and power as the dungeon goes on, but the game is rigged. Nobody has ever survived beyond the 13th floor.
This season with infighting between the alien governments, the AI slowly going a little batshit insane and one particularly determined crawler and his (now sapient) ex girlfriends cat. Who knows?