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A hallmark of any game in the series, Basim must eliminate targets throughout Assassin’s Creed: Mirage, one of those being Al-Mardikhwar. This target is one of the Order at the end of the multiple Investigations that began at the Sharqiyah Bureau for the Hidden Ones in Baghdad. Finding and eliminating him will take some creativity from Basim, as well as help from the Rebels.
Among other assassination targets such as Fazil the Great Scholar, Basim will have to find a way to quietly eliminate Al-Mardikhwar. Basim and Ali ibn Muhammad approach the Great Garrison in order to find and free Beshi, along with other captured rebels. They must infiltrate and find the Warlord Wasif, Al-Mardikhwar's real identity, to end the mission.

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Getting Into The Garrison
To enter the Garrison, use some eavesdropping and careful planning to move into the fortress and meet the Steward. There are other ways to enter the Inner Garrison, but this one will lead the player on the most direct and quieter path. To do so, look for the archer standing near one of the main entrances.
It is possible to approach and sneak behind him, assassinating him if you so wish along the way. This will get you into position to drop hidden to a bit of wall that Basim can slip under and into the main part of the fortress where the Steward is located.
Once inside, use the Eagle Sense to spot where the Steward is waiting by the cookhouse. By moving upward directly from where Basim entered, it is possible to sneak past the Guards and make your way to the back where you can approach the Steward unseen. Talk to him, and he will direct you to follow him by staying in the shadows unlike when Basim stayed directly beside the Merchants when infiltrating the Damascus Gate Prison.
Assassinating the guards at the entrance, including this archer and the other main guard nearby, might be seen by civilians, causing Basim's Notoriety to increase. Distract or sneak past them to avoid this, or take the risk. One level of Notoriety will not affect the rest of the quest and it can be lowered as soon as you have finished.
Trailing the Steward
Trail the Steward through the fortress, staying in the grass and edging along the wall and up high where necessary. He will lead you and wait for you to make it where you need to go before moving forward. Taking out the spearman who is watching the row of guards as they have an inspection is necessary for Basim to continue moving along sneakily. He can sneak behind the archer, but if he does not assassinate or distract him, the archer will spot him as he tries to move beyond that point.
Continue following the Steward, quietly assassinating anyone that he must, although it is possible to distract the guards enough to move along without doing so. He will open the door to the Inner Garrison, and Basim can slip into the room, where he will be left to his own devices once again. Move through the quest door and a cutscene will trigger.
Freeing the Rebels
After the cutscene, Basim and Ali must take out the guards who attack them. Doing so will clear the courtyard, and entering the prisoner hold will inform Basim that he needs to take a key from the Warden. Go back to the courtyard and slip up the ladder and stairs until you reach the point where the Marksman is waiting. There is an archer up there as well, assassinating them both to clear the path as this will also be helpful later and will allow Basim to use Enkidu.
After that, Basim can sneak along the roof until he finds the Warden at the end of the path. There are multiple ways to take out the Warden, but using a Blowdart to send his accompanying guard into Berserk mode will be particularly useful. Wait for the other Guards and the Warden to take out the companion, and then assassinate the Warden once he is alone. Travel back along the roof and re Ali to free the rebels. Ali might find himself fighting Guards again, so take them out if you must.
There are also several red fire pots around, which can be used to distract or kill some of the guards as well.
Getting the Warlord Alone
After freeing the rebels, protagonist Basim in Assassin's Creed: Mirage will watch Ali talk to the rebels in a cutscene, after which he will be tasked with lighting a signal fire. This should be fairly straightforward as the courtyard should be clear. Light it and wait for the rebels to take out the guards who come to the fire, before sneaking back to the roof where you previously cleared off the Marksman and Archer.
The room Basim needs to sneak into is below that, and depending on how you eliminated the Warden, there may be three guards outside of it and one inside the room. Wait for an opportunity to sneak by or take the guards out one by one. Basim can fight the guard inside the room one-on-one if he has to, as that guard faces the only entrance. Then unlock the trapdoor in the room and another cutscene will play and Basim will drop into the room below.

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Eliminating Wasif
At that point, sneak to the end of the hallway and watch from a high point as Wasif informs his guards to send off a letter informing someone that they need reinforcements. Wait for those guards to leave and then cross to the other balcony using the swinging chandelier.
Using your Eagle Sense should reveal the point Basim is trying to reach, but stay up high to get the most benefit. From there, trigger the assassination, and the cutscene will play, showing that Basim has now successfully taken out Al-Mardikhwar.
At that point, Basim will be forced into open combat with Ali and the other rebels. You have a lot of help at this point, so it should be fairly straightforward, particularly if you are using one of the best swords in Assassin’s Creed: Mirage. Leave the Garrison and regroup at the Sharqiyah Bureau to complete the quest and continue Basim’s journey.