The Greater Good is the final Main Quest in Assassin’s Creed Origins – The Hidden Ones DLC. This walkthrough shows how to complete the quest.
Recommended Level: 45
Region: Arsinoe Nome
Requirement: having finished Sic Semper Tyrannis
Reward: 15.000 XP + Legendary Light Bow
Trophy: 
The Greater Good
Objectives:
- Speak to Amunet
- Follow Amunet to the rebel camp
- Explore the Rebel Camp for Gamilat’s whereabouts
- Find & Speak to Gamilat in the Sunken Quarry
- Kill Gamilat
With Rufio finally gone, it’s time to inform Aya about it. She is certain that the threat for this region is not quite over and opens Bayek’s eyes. Gamilat, the rebel leader has provoked many fights with the Romans, which led to many killed civilians and the remaining family members eager to avenge their deaths and join the rebels.
Bayek cannot believe that he has been blind to this and wants to confront and talk to him.
Gamilat won’t be present at his camp. Go talk to the man sitting close to the top of the camp – he will reveal that Gamilat is out in the Sunken Quarry.
Gamilat’s whereabouts are not too far away and we will find him sitting on a large wooden structure which emerges from the water.
Bayek can now finally confront him and because they cannot agree, the inevitable fight will start.
Just like any other boss-fight, dodge 
and counter-attack quickly. At some point, he might push you off the structure and set it ablaze. You won’t be able to climb on it for a while and he will try to shoot you with the mounted crossbow.
If you have a bow with fire-damage, use it to your advantage. When Gamilat burns, he will leave the crossbow and runs enraged towards you.
Again, dodge and counter and make use of the overpower-attack 
+ 
as this deals a lot of damage.
If you can set fire to him every now and then, he will go down even faster. As with the other leaders, you will need to confirm the kill.
In the dialogue, Gamilat admits the mistakes he made, accepts his death and hopes that the Brotherhood’s Creed will live on forever.
After this sequence, a few other cutscenes will play and Bayek will ask the ultimate question:
Dan says
there are two safe places you can sit and pick him off with arrow shots:
at the base of the stairs in the water opposite him
a full quiver of 10 with a maxed bracer is enough to kill him diving deep to avoid the crossbows when changing locations