Rewards: Pounds: 3480 (+250), EXP: 4000 (+250), Extra: Outfit - Blackguard's Suit
Challenges: 1. Do not kill a single policeman. 2. Do not get detected by the target.
This mission once again has a unique kill opportunity, a stealth opportunity and an assist opportunity. The unique kill opportunity can be triggered by stealing a password from a special police officer (marked, as usual, with the skull icon). The stealth opportunity requires you kidnap the corrupt policeman in the center of the garden. Lastly, the assist opportunity has you follow a politician inside the building, who'll lead you straight to the door where the password needs to be used.
Let's begin with getting the unique kill opportunity. Since there's also a challenge that states you cannot kill any policemen, climb onto the roofs and make your way to the skull marker. There are various policemen down below, including your target. Although it's definitely possible to drop down behind your target when the policemen are looking away, this is rather tricky to achieve. It might be better to first knock out several of the policement down below by carefully approaching them from behind, strangling them (not killing them). With the coast clear, approach your target from behind (if necessary, first inspect his patrol route beforehand). Steal the password, then strangle him as well.
Your next goal is to kidnap the corrupt police officer. In fact, you are still able to kidnap him if one of the police officers' suspicion levels are yellow. In any case, kidnap him by approaching him from his back, then approach the main entrance and carefully and slowly walk past the two guards, who automatically step aside as you move closer with your kidnapped officer.
Inside, strangle the officer and approach the nearby politician. Speak to him, then follow him. There's a brute in the corner of this room that you'll want to take out with a knife headshot to avoid trouble. Should the politician move into a 'scared' stance and stay still forever, bump into him to have him continue. It's fairly smoothsailing from here; you'll come across several enemies that are approachable from the back, so use the opportunities for (double) assassinations. As you move into the next few hallways, more Blighter and Templar enemies emerge. Either ignore them if possible, or take them out (smoke bombs may help with large groups).
When you reach the final door with the politician, use the password automatically and assassinate the Earl of Cardigan. Afterwards, all you have to do is escape the building, which isn't too difficult. Simply head the way you came and climb the building, then move via the roofs to the streets. The premises are large, so take your time.