How do I get into the arena fight?

In Assassins Creed Origins how do I get into the arena fight? Is it unlocked after a certain part of the story?

So, full disclosure, I do not own nor have played AC:O.

However, after a quick search, it seems like this guide may be what you are looking for if you are looking to unlock the Krokodilopolis arena. Additionally, here is an excerpt from a second page that corroborates the first guide, and both indicate that a story mission takes you to Krokodilopolis (emphasis mine).

Krokodilopolis Gladiator Arena is found in the Faiyum Oasis (level 28-31 zone), in the southern part of the map. To unlock this arena, you’ll need to complete a side quest called The Champion first. This quest can be found in the northwestern part of Krokodilopolis, and it’ll task you with helping an aging gladiator. There is also a level 28 main story quest called The Crocodile’s Jaws that takes you straight into this arena. Completing this one might also be a requirement for entering the arena.

Here is an image of the location of The Champion sidequest which unlocks the arena.

Unlocking the second arena in Cyrene appears to be tied to completing challenges in the first as well as another related sidequest.