Upon registering for the Battle Base challenge, players are required to surrender all items except for Mega Stones and Z-Crystals. They then are brought from the lobby to the main hub area. From this room, they are faced with five door options -- going directly north leads the player to the password gate room, while each door to the left or right leads to a different wing.
When a player chooses to enter one of the wings, they will have a choice of two pathways. The simplest path is to proceed directly forward (east or west depending on wing orientation), where the player will be greeted with a battle. Winning this battle allows them to proceed and learn one of the four passwords. Under normal circumstances, these passwords are random and unrelated to each other, but sometimes they are specifically chosen to form a phrase (e.g. "Sweat Inducing Training Session").
Should the player choose to explore a bit, they may choose the other path (north or south depending on wing orientation) and proceed to the wing's puzzle room. This room provides may be any number of puzzles to solve -- warp panels, ice sliding puzzles, Rocket Base spinning panels, just a plain walled maze, or other possibilities. These will always have two exits, with a warp to the entrance in between them. One of the two exits will always lead to a free item; the other, always to a battle of some sort. The nature of this particular battle is not always a standard trainer, and may instead be an Electrode or Amoonguss disguised as an item, who will proceed to attempt to debilitate the player's team through status conditions that will remain until the player finds something to heal with.
When the player returns to the central areas, they do not need to retrieve all four passwords; simply retrieving one password is enough to convince the guard at the password gate to let the player through. However, the guard will reward those who continue to recover more; by providing two passwords, they will heal a random one of the player's injured Pokemon; three passwords will heal two random Pokemon; and four passwords will fully heal the player's entire team. If the player has not actually collected any passwords, they may still continue through, but must first battle the guard instead. Like the previously mentioned Electrode and Amoongus, the guard's duty is not to attempt to defeat the player, but rather to debilitate them as much as possible so that they are unable to fight at full capacity in their final battle.
After the guard's test, the player may finally proceed to the boss room. Except in special circumstances, a player completing the challenge for the first time will not fight Kai, but rather, a designated Raid Admin. The admin will be a particularly skillful, but nonetheless randomized trainer. On the player's second successful challenge, they will finally have an opportunity to battle with Kai. Unlike the admin, Kai has a particular strategy, taking advantage of the damage and status conditions the player has suffered throughout their raid.
Kai will reappear when the player has a streak of five wins, using a team including Nix.
The concept of the Battle Base was first considered when Kai, alongside Anabel and the J-Team, raided Best Berry's facility to retrieve the Mewnium Z and Team Umbra's collective Pokemon. It was not brought up again until April Fools Day 2018, when Kai pranked Tagg into test-running a beta version of the facility located in an abandoned warehouse in Petalburg Woods. Echo helped plan the layout and format, and lent old Illusion Project outfits to the Frontier trainers who filled the roles of the "grunts".