Start this one by creeping into the caverns toward the south of the map, by the makeshift arena.
The cutscene introduces you to a new threat and an old frenemy. The threat is the Olog-hai, supersized orcs that you can ride - a bit like giant goons in the Arkham games.
There's only one here, so no need to get fancy. Headshots are an easy way to stun him, but even basic swordstrikes will get the job done. When he goes down, follow Gollum to the next objective (something you'll be doing a lot of in the near future).
This leads to another chat with Shelob, who's still just chillin'. Her vision will set you on a quest to take out an orc assassin before he finishes work here and heads to Minas Ithil.
Follow Gollum back out toward the assassin' first victim. Along the way you'll stop for a brief tutorial on shooting Morgai Fly Nests to terrorize local orcs. You don't need to charge your shot- any quick poke will do.
Just past this courtyard Gollum will stop, and you'll need to enter the Wraith World to find the body of the victim. Follow the glowing footsteps to the ex-orc.
Search the body for a bit of CSI:Cirith Ungol, as you learn that *only* morghul blades could have made these wounds. When done, follow Gollum to the next stopping point, and again go Wraith to follow the trail. Note that the trail may take some unexpected directions.
Search the body and follow Gollum. Along the way you'll be forced to dispatch a patrol. Luckily, they are all facing away from each other, so you can stealth kill the lot of them, then finish up with the Olog-hai by the time-honoured expedient of shooting him in the face.
The next and final stopping point has a few false leads- just follow them until Cel comments about how you're on the wrong trail, or the assassin has doubled back. Eventually you'll find the final victim, still clinging to life.
Interrogate him for the goods on the assassin. This works just like taking out any other captain- note his weaknesses, find him with Wraithsight, and then home in on the kill. Note that there is a bonus for using the assassin's fears against him. In this case he's afraid of the Morgai Flies- open the fight by popping one of the nearby hives.
After that it's a standard fight. The flies daze him, so close quick and pummel quicker, before reinforcements flood in. Once this unhired killer is down, the mission is over.
Note that completing this mission unlocks the Death Threat skill, allowing you to raise the level of enemy captains in order to increase the value of their loot.