Your main quest objective here is to assassinate the infiltrating homunculi. The full breakdown of objectives is as follows:
Let's go over each smaller objective and how you should go about completing it.
Your Silvermoon Mark is Tercin Shivenllher.
From Dalaran (Legion) take the portal to Dalaran crater (have your Safe Fall ready). From there, fly to the Eastern Plaguelands and take the instance portal to the Ghostlands, then run towards Silvermoon City in the north.
Tercin and his bodyguard, an Arcane Demolisher, walk around the city in a loop. Distract or Blind the Arcane Demolisher, so it separates from Tercin. Otherwise, it will one-shot you if you try to kill him with it still around.
The loop path they walk is depicted below:
Your Undercity Mark is Mistress Matilda.
Head to Dalaran (Legion) from Silvermoon and take the Dalaran Crater portal again (have your Safe Fall ready). From there, you just have to fly across the lake to the north to get to the Undercity.
Go into the city stealthed, and make your way to the canal area of the Rogues’ Quarter. Find a safe spot to mount up and fly. You’ll see a balcony where a party is going on.
Matilda goes back and forth, mingling with the other partygoers. There are Elites at this party, and they hit like trucks. The best spot to stake out is in the center of the balcony, near the buffet table. Some of the Elites will make their way to the table for refreshments (even the Forsaken need some grub!), so keep an eye out for them when you attack. Take out the non-Elites when there’s a window and wait for your opportunity to take out the mark.
Your Thunder Bluff mark is Kroksy the Impregnable.
There are several ways to go about getting to Thunder Bluff. They break down as follows:
Kroksy is located on the main bluff, near the city leader Bane Bloodhoof (Coordinates: 56, 51). He is standing right by the unlit bonfire. Flying is the best way to get to him. Land in a safe spot and stealth.
He is stationary, but has a buff that reduces the damage he takes by 99%. Pickpocket a key from him, then Sap and use that key to take his armor off to get rid of the buff. Once you do, he will run away from you and cast Cry For Help to call for guards -- so make sure to interrupt him and take him out quickly.
Helpful Macros for This Mark:
Your Orgrimmar Mark is Auctioneer Xifa.
To get to Orgrimmar, fly from Thunder Bluff. Or if you want to be ballsy again, take the Zeppelin.
Xifa is located in the Auction House close to the front gates to Orgrimmar. She is on the far right of the auctioneer platform. The Auction House is always busy, so expect there to be enemy players ready and willing to kill you.
Since there is an invisible wall behind all the auctioneers, you cannot Shadowstep or Shadow Strike her. You will have to run up to kill her, then Vanish if you are able.
Beware. There is a stealth-detecting guard that pats into the Auction House -- so keep an eye out or you're sure to get caught.
Once all the marks are dead, head back to Lillian and reap your rewards!