The Treasurer Investigation is one of a series of investigations assigned to you in Assassin’s Creed Mirage. The Treasurer becomes your target once you’ve successfully completed the previous two investigations, namely The Harbourmaster Investigation and Of Oil and Taxes Investigation. You must complete The Treasurer Investigation first in AC Mirage after which you can take him down.
This comprises three cases: A Faceless Feather, Gilded Butterflies, and A Grand End. While the main essence of the assassination is the Gilded Butterflies quest, you will have to pursue the other two to start and complete the investigation.
A Faceless Feather Walkthrough

Having completed The Harbourmaster and Of Oil and Taxes Investigations, you will return to the Bureau to meet with Rebekah and Roshan. After knowing about your success in the previous missions, they will decide that you are ready to pursue another assassination. This time, your target is the Caliph’s Treasurer, who is also closely connected to Al-Anqa and Al-Kullulu.
Roshan will ask you to meet her at the Bazaar to discuss further about the matter. Once there, she informs you about the action plan in The Treasurer Investigation in AC Mirage. In short, you will get the specific hairpin our target wants to acquire and use it to have an audience with them when the appropriate time comes.
As you end the chat with Roshan, you will be able to see the assassination information screen. Everything about the background of The Treasurer will be mentioned, except their true identity.
Gilded Butterflies Walkthrough
After the meeting, drop down the Bazaar tower and head to the central area crowded with merchants. A Faceless Feather quest ends here with a new quest at hand, “Gilded Butterflies”.
From the conversation at the Bureau and the Bazaar, it is clear that The Treasurer is situated at the Great Auction. However, we are still unable to know the precise identity of our target, for which we still have to do some workaround.
Look for Intel in the Bazaar
The first thing you need to do is learn everything you can about the Treasurer from the Bazaar. Go left from the entrance of the Plaza and meet your old friend, Kong, in The Treasurer Investigation in AC Mirage. As you ask him about The Treasurer, it will be revealed that the so-called friend won’t give some free information. You will have to fulfill his request first for him to do yours.
Get Kong’s item from Luca

Kong tells you that Luca has taken a precious ceramic saucer from him out of jealousy and needs you to bring it back to its owner. Luckily, our friend didn’t forget to tell us the exact location of what he wanted us to find for him. The rest of the ambiguity is cleared with the help of the quest marker.
Follow the quest marker until you reach Luca’s stall. You will discover a breakable wall at the entrance (left from the door) as you enter. Push the rock-and-wood shelf to the other side of the room and use it to climb the upper area. Carry an explosive pot and throw it exactly on the breakable wall. This will produce an explosion, destroying the wall and creating a new opening for you.
Get Kong his Stolen Saucer

Bring the shelf in the newly discovered area with you and use it to climb the upper part of the room. There, you will spot the saucer that Kong asked for. Pick the plate up from the table and exit using the door on your right.
Please make your way back to Kong and give him his saucer. Eventually, he will hint about two ladies speaking about something they’ve made for the treasurer. You can leave Kong’s stall and progress the investigation further.
Visit the Foreign Merchant’s Stall
While in the Bazaar, you can go upstairs to visit a merchant loudly bragging about his Chinese artifacts collection. There will be nothing of interest there except the information that the hairpin can be acquired at the Auction after paying a ton of money.
Eavesdrop on the woman’s conversation

When you’re done with the merchant, go outside and approach the mysterious woman on the opposite floor. Use the narrow path going above the tomb to get there. If you find it difficult to locate her, use your Eagle Vision to spot her on one of the upper balconies. The orange character will be the one you are looking for in The Treasurer Investigation in AC Mirage.
Approach the woman’s location and sit silently on the bench beside the wall to hear what she’s talking about with the Perfume Supplier. After a moment, they will leave the area.
Follow the mysterious woman
Your next task is to follow the woman’s path and keep hearing what she says about the Treasurer. When they both reach the woman’s (Qabiha’s) quarters, blend in. Sit at the table to spy on the conversation like you did before. When the perfume supplier leaves, you need to speak with Qabiha and ask her about the perfume they were talking about.
After the conversation, you will finally learn that The Treasurer is a woman named Ning. You can now head to the auction and get your hands on the hairpin.
Attend the Great Auction
Keep following the new quest marker until you reach the Great Auction area in The Treasurer Investigation in AC Mirage. Blend in and listen to the host to know what offerings are here to be auctioned. You will watch Ning arriving at the auction and bidding for the hairpin.

However, a bearded man will pay more and acquire the hairpin instead. As Ning isn’t interested in anything besides the hairpin, she will immediately leave the area with disappointment.
Steal the Hairpin from the Merchant
Go to the upper floor from the auction area and approach the merchant standing opposite a man gazing at an ancient artifact. Go past him stealthily and steal the hairpin from its position.

Don’t make the mistake of staying in the same place for too long. You must go to the area immediately by jumping down to save yourself from being caught.
Speak with the guard
Follow the quest marker to reach The Treasurer’s chambers. The guard outside the door will argue with a merchant about his entry to Ning’s chambers. Once the merchant is asked to leave the site, speak with the guardian. Upon asking, he will inform you that entry is only possible if you have an Ivory Coin Brooch.
Bribe the Merchant
It’s time to reach the Spice Merchant on the opposite side of the guarded door and ask her about the Ivory Coin. She will be hesitant initially, but 150 Dirhams (or a Merchant Favor Token) will remove her hesitation.
She will inform you that only some respected merchants are part of the Ivory Coin, so the Brooch isn’t legally accessible to ordinary people.
Find (and steal) the Ivory Coin’s Brooch

After concluding the conversation with the Spice Merchant, you will have to go back to the entrance of the Bazaar. Go past the guards there without any noise to reach the Restricted Area. Make your way inside and use your Eagle Vision to know the location of the guards in a specific room. That is also where the Ivory Coin’s Brooch is placed.
Carefully go inside the room and distract the guards with a whistle. Stay behind the pillar until one of the guards starts moving toward your location. Immediately throw a knife at him to remove the remote guard and kill the one marching toward you. You can also lure them by hiding in a changing closet and killing them individually.
Having dealt with the guards, approach the desk and steal the Ivory Coin’s Brooch from the pillow.
Assassinate Ning, the Treasurer
Head to The Treasurer’s chambers to gain an audience with her. The guard will happily let you in as you have the Brooch.
A Soldier will guide you to Ning’s chambers. At the end of your talk, he will ask you to refrain from speaking anything appropriate with The Treasurer.
Once you enter Ning’s magnificent resting place, press the Interact button to trigger a long cutscene. During the cutscene, Basim offers the hairpin to Ning to gain her trust for the moment. When you regain control and get the option to Assassinate, go along with the prompt to initiate combat.

Ning will sense your intentions and try to do the attack first. However, you will eventually end her life using your quick attacking skills. And that’s it; the world has gotten rid of another member of The Order.
A Grand End
After assassinating the Treasurer, you must leave her chambers cautiously as her defenders guard the area heavily. It’s time to return to the Hidden One’s Bureau and inform Rebekah about your success in The Treasurer Investigation mission in Assassin’s Creed Mirage.