The Arena is the main location of the mod, and the main "attraction" of Vault 5. It is where most of the action takes place, as the player needs to spend a minimum of five nights in it in order to complete the quest.
It was designed by Vault-Tec as part of their secret social experiment, which was meant to test how well can the selected portions of the populace fare when faced with various, purposefully designed stress scenarios. The Arena was a more complex experiment than most, facing the inhabitants with an obviously deadly survival training program that was obligatory to attend - every night someone had to go inside, or else the Vault's systems would start emitting lethal doses of radiation and potentially kill everyone inside.
The Arena is designed to demand a participant every night until all of the five nights are beaten, in which case the Reward Room would be unlocked and the obligatory attendance rule would be lifted, allowing the inhabitants to safely live inside the Vault. However, if anyone dies while making their way through the nights, the counter is reset and the next person to go in has to start from Night 1.
The Arena is inhabited by a grand total of 5 robots:
There are also other dangers to look out for, like the terminals and the doors, which are a potential source of radiation.
The player has to survive from midnight to 6 AM in order to win a night. It is necessary to enter the Arena between 22:00 and 23:00, in order to have enough time to prepare oneself for the night.