Halo of the Huntress (Side Quest)
This quest begins in the Oracle of Apollo, in the Green Mountains.
Right away, Bayek has a good feeling about this.
Speak with the Oracle herself to be granted a divinely ordained quest of treasure seeking.
Set course for your marker, but either ride or warp directly to the fast travel point nearby.
Leap-of-faith on down to discover the lake of tears.
The next part of the prophecy leads you to the Collis Roman Hunting Camp, on the far shore.
Note the treasure in the near (east) tent, and the high probability of animals rampaging on the loose.
There's a gap in the fence close to that tent which will get you inside camp.
The treasure itself is in a red chest there.
The random animal and bandit attacks can make orderly tactics irrelevant, so just snipe the captain when you get the chance.
This completes the location.
Snipe anyone else in your way that isn't already being maimed by animals, and descend into the ruin.
Smash through the cracked wall below.
There's a lot to loot and read here, but the Halo itself is just in front of the central statue. Claim it to complete the mission.