This one begins at the end of Ulterior Votive. Menehet asks you to search for his missing star- use Senu to track him down in the marketplace.
When you find him, he's not in good condition.
Pick him up and carry him to Menehet.
Since he's in no shape for ceremonial fighting, Bayek volunteers. Speak to the High Priest to seal the deal
Walk into the arena and fight.
This is a surprisingly tough match, but it's not fatal- if you 'die' it just starts up again, with both fighters healed (though your adrenaline meter retains its charge). You can't use anything except the spear and shield provided, and for a ceremonial fighter this Isfet character is extremely gung-ho. In any case, the key to taking him down is maintaining a good distance. Your spear doesn't do any extra damage at point blank range, and if you're too close you won't be able to dodge his attacks in time.
You'll also want to keep your shield up. Not only will it deflect attacks, but he's oddly less cautious when your shield is up, and easier to outmanuever. While you can get a few good whacks in every time he misses with his charge, a more reliable way to do damage is break his guard with a strong attack, follow up with two light attacks, hit him with another strong just as his shield is coming up, and repeat. Do use your adrenaline attack when you get it, but remember that you still need to break through his shield for your hits to hurt him.
Keep stacking up the hits, and dodge out of range when his own weapon starts glowing, and you'll grind him down eventually. Final victory completes the mission.
You also get to hang onto the Sekhmet costume and weapon- the perfect thing for discreet assassinations.