When reaching Apocalyptic or higher on your combo meter, the next non-challenger layer a bobblehead resembling a known figure will appear.
You must melee this bobblehead.

Repeat this 3 times and the main fire will turn green. This will commence the main part of the quest.

Notes:
1. Currently there's an FX issue if you melee the bobblehead AFTER the portal has spawned, the portal won't turn green, you will however still enter the secret level.
2. BO3 has an issue with high frame rates. If a jump seems impossible and you play above 144 FPS, please cap it to 144. We'll be adjusting this level in the future to accomodate for this.

While inside of this secret level, a certain scripture hoists a unique secret...

Become V1, a superior machine. V1 will change your look and provide you with a 25% combo bonus and 25% boss damage bonus.

If the secret of the scripture has been found, the entity found inside of the scripture will become available in the store.

From layer 8 and up, a blue glowing character will have a chance to spawn in the audience of the colosseums.
Shoot the character and force them to vanish.

A key will now be able to spawn in your store. Purchasing this key will unlock a unique challenger, who'll appear in the store. (They're pretty strong... be prepared)

Be able to challenge Mephiles, a unique boss fight with its own rewards. Once beat, Mephiles becomes repeatable.

After defeating all challengers (3), a new challenger in yellow highlighting will appear in the store.
This is the Final Challenger of the colosseum, challenging them is a point of no return.

Conquer the Chosen One, and you will have successfully beat the colosseums.