The translator explains that the Big One is effectively the leader of the lizards. It is very old, very powerful, and frighteningly intelligent. When the people first came to these tunnels to escape the unending winter outside, they displaced the lizards from the hot springs, and ever since they've fought to reclaim it. For the most part, the lizards did not have much success, occasionally taking down a small group of people who stray from the main caverns, but never organized enough to present much of a threat. Then, one day, a very long time ago, the Big One appeared. Ever since, the war has gone badly for the people. The lizards are now somehow organized and brutal.
It's capacity for strategy has given the lizards an edge, and the fighting has been fierce. It got to be so bad for both groups that a truce fell between them two generations ago, but then the fighting renewed after about a decade.