The arena.
It exists in every era, even in a world of wasteland that is on the verge of collapse.
When a force reaches the scale of a "base" it signifies that "survival" is no longer the biggest dilemma. People within the base can resume some semblance of life, especially the elites.
As such, entertainment emerges as a necessity, and the most popular form of entertainment on the wasteland is undoubtedly combat.
Here, spectators exchange goods for currency to buy tickets, witnessing battles that are entirely composed of the Tainted Miasma, either against monsters or amongst each other in bloody combat.
The best way to support a favored contender is through betting, with victory in battle yielding rewards based on odds.