The Scarab's Lies (Main Quest)
This quest becomes available in Letopolis after completing 'The Scarab's Sting,' Ride to that sandswept city, and speak to its overseer, Taharqa.
You'll retire to his home to speak further, but are interrupted by a sudden alarm- bandit attack and sandstorm combo.
Follow Taharqa into the storm to defend the city.
As ever, your NPC ally can be downed but not killed by the enemy, so don't worry too much about his health- instead focus on killing bandits.
Taharqa will lead you through several skirmishes. Note that, between the storm and the city defenders, you can often just run right up to combatants and Assassinate them.
The final such fight takes place just as you're exiting the temple. This band features the bandit leader, the only one so far to carry a full shield, so adjust your gear accordingly.
When they're all dead, speak to Taharqa.
He wants to immediately carry the fight back to the bandit base. You reluctantly agree. Follow him out of the city and summon your mount, or grab one of the many available.
The warpath leads to Plesionhudor Hideout.
If you want to complete the location while you're here, you can kill the captain in the mad melee, but the chest is in the central tunnels, on the lower level, in a red box (the same level as the captured priest, but one room west)
Once you've looted it, kill any bandits left standing to complete the current objective. You might want to have a Predator Bow handy for any that try to flee.
When they're all dead, follow Taharqa back through the cave mouth to the captured priest, and speak with him there.
Free the priest. After some brief natter, your next objective will be to meet Taharqa at his home, back in Letopolis.
Things go badly, and you wind up in the sands.
Enjoy the hallucinations until you get the prompt to summon your horse.
You'll be pulled free with a bit of teamwork, but your gear is still being held at Camp Achlys (location guide here), so you'll have to ride over to retrieve it.
Senu will pinpoint your gear as being held in the ballista tower.
You still have your hidden blade, so sneak up via the scaffolding, and ice the door guard from above.
Quickly drag his body inside, then open the chest to regain your gear, and get a plot-important letter.
Now it's time to kill the Scarab. You can find him back in Letopolis, performing rituals in the temple.
Start at top- ascending to the roof via the ladder in the northern corner.
The guards are pretty spread out- use the block of timber here as cover to lure over and assassinate the guards one by one.
After all that, killing the Scarab is simple- an arrow to the back of the head won't even trigger an alert.
Confirming the kill is trickier- there are plenty of guards around the periphery who might catch sight of you. Lessen their number by getting back to ground level via the 'assassin's feather fall.'
Leave this body where it will be found- you don't care about commotion- you just want people looking at something besides you. Snipe any obstinate stragglers inside the temple, then confirm your kill.
This begins the usual afterworld mediation, then a more tragic real-world epilogue.
It is all a little odd- Taharqa was the feared Scarab, but didn't control either the sandstorms or the bandits? Had to use a volunteer workforce and couldn't even requisition a single warrior of bandit-clearing power? Was he just an intern with the Order? In any case, the end of the cutscene is the end of the mission.
This immediately throws you into the main quest Pompeius Magnus.