Tribunal:An Assassination Attempt

Quick Walkthrough
#Speak to King Helseth in the Royal Throne room to receive your orders to investigate an assassination plot.
#Talk to the Orc Bakh gro-Sham in the Winged Guar in the Godsreach area using the code phrase ''uncle's farm''.
#Return to Helseth with the news that Queen Barenziah is the plot's target.
#Travel to Barenziah's room at the top of the Imperial Cult area within the Royal Courtyard.
#Close the door to Barenziah's bedroom and the door to Imperial Cult Services. Then hide behind the screens until around 9 p.m. the next evening.
#Intercept and kill the three Dark Brotherhood Assassins.
#Return to King Helseth for your reward of Helseth's Collar.

Detailed Walkthrough
King Helseth
After investigating the disturbance for Tienius Delitian, you are told to speak to King Helseth in the Royal Palace Throne room. He slothfully apologizes for the little incident with the Dark Brotherhood as it seems that someone mentioned that you were vying for the throne. The King certainly doesn't seem to take the threat of the Nerevarine lightly, but plays down the matter. After this little bit of unpleasantness Helseth will ask you to look into another possible assassination plot. You are to meet with an Orcish informant in the Winged Guar in Godsreach by using the code phrase ''uncle's farm''.

Winged Guar
The quickest way to the Godsreach area is through the Royal Courtyard, to the Plaza, and West to the Godsreach entrance. The Winged Guar is the Northern building in the center of the area. Bakh gro-Sham is the only Orc in the place and will respond when you mention the code phrase, though you may need to raise his disposition a bit to get him to admit to recognizing the code. Although Bakh thinks that the king is paranoid, he believes to have found a plot on the Queen Mother Barenziah's life, without further information on who exactly is involved or why.

Back to Helseth
With this sparse bit of information, return to King Helseth, who says it confirms what he has heard from his other informants. He now wishes for you to protect the Queen from these assassins and goes on to tell you exactly how. Helseth doesn't want to station more guards around the Queen as this would alert the assassins. Further, some of the guards may even be involved in the plot so this might actually increase the danger to the Queen. Instead, you are to be the sole guard outside the Queen's chambers, hiding behind a screen and deal with the killers whenever they show up. Speaking with Plitinius Mero at this time will give a quest update regarding his doubts to Helseth's true intentions.

Queen Barenziah
The Queen's chamber can be found through the Imperial Cult area at the back of the Royal Throne room. You may speak with the Queen though she is simply 'unconcerned' about the plot to kill her. The screen Helseth mentioned is just outside her chamber and is conveniently slightly transparent so you spot anyone who tries to enter the room.

The Killers
You most likely will not encounter anyone for some time as Helseth only mentioned to expect the assailants the next night. You also must make sure that the door to Barenziah's bedroom is closed and the other one is closed as well, as to not alert the killers.  You will receive a journal entry when you are in the 'correct position' to intercept the assassins. The best bet is to save and then wait for a few hours for the killers to appear (they should come in around 9pm). The light will go out and you hear whispers and grasp that the real target is you. The door to the Queen's chambers is now magically sealed, leaving you trapped behind the screen. No matter, there are three Dark Brotherhood assassins who have a good deal of hit-points. They, however, don't hit very hard and are not magic resistant, so taking them out shouldn't be very difficult. You'll receive another journal entry when you've killed them all.

Return to Helseth
Return back to the King to report your success in "protecting the Queen". He seems surprised that you succeeded and rewards you with Helseth's Collar and an offer to be of further service to your king.

Notes
Don't be surprised if you do not initially receive the journal entry indicating you are in the correct position. If you are behind the screens and both doors are closed, then simply wait until the evening and your journal should update at the same time the assassins enter the room.
It is revealed during the quest that the whole plot was a lie from the very beginning, and the assassins were sent not after the Queen Barenziah, but after the player character. Another test from paranoid Helseth, who obviously broke his promises not to use Dark Brotherhood against the player. When assassins appear, and you are still hiding, you can hear the whispering voice of one of them: ''he told us: in a Queen Mother's chambers...behind the screen...
Before you deal with the assassins, you can ask Plitinius Mero about the plot, and he confirms that it is indeed a fake by saying "A plot against Barenziah? That's ridiculous! I can't think of anyone who would want to harm her. Take care with this friend. King Helseth is a wary one, and delights in testing the loyalty of his new friends. I do not believe the King would hesitate to put his own mother in danger in order to do so."''

Quest Stages