Astrid informs you that the time has come to undertake the mission to assassinate the Emperor of Tamriel, as was arranged with Amaund Motierre, the contractor the Night Mother referred to you.
The plan is as follows: you will travel to Castle Dour in the city of Solitude. You will approach the officer in charge of the Emperor’s security force, Commander Maro, and introduce yourself as the Gourmet. You will show him the writ of passage you took from the real Gourmet to gain admittance to the castle. You will prepare food for the Emperor, and add to his meal a poisonous substance that Astrid will have given you. If for any reason the plan to poison him fails, you will have to attempt to kill him with a direct attack. You will escape the castle via a rear exit immediately after you know he is dead.
After receiving the poisonous substance from Astrid, leave the headquarters and journey to Solitude. Make your way to the sprawling Castle Dour that sits atop the hill. Step into the large courtyard and you will find Commander Maro waiting by the tower entrance. Maro is the head of the Penitus Oculatus, the Emperor’s special security force. Present the writ of passage, and he will permit you to enter the castle through the door behind him.
Once inside, you’ll meet a young woman named Gianna, who works on the Emperor’s cooking staff. After telling her you’re the Gourmet, she’ll ask you to wear a Chef’s hat if you are not already wearing one. There are some on the shelves to your left. Go over and put one on, then return to Gianna.
You and Gianna will now prepare the meal for the Emperor. You can tell her to use whatever ingredients you like, as long as you eventually tell her to add the Jarrid Root, the poison ingredient you got from Astrid.
When you’re done, Gianna will take the meal to the Emperor. He will drop dead shortly after tasting the meal, and his guards will become hostile. If your sneak skill is high enough, hiding in sneak mode while the Emperor is eating the meal could reduce or prevent any confrontations with the guards. Once the Emperor is dead, escape through the back door.
As you escape across the bridge, you’ll be stopped by Commander Maro and three of his men. He tells you that the man you just killed was a body double. He then explains that someone within the Dark Brotherhood made a deal with him that he’d get to apprehend you, and in return he would cease all efforts against the Brotherhood. He finally reveals that he intends to break the agreement and kill everyone within the order after he’s killed you. He’ll then set his men upon you. You can flee from them or kill them.
Afterwards, hurry back to the headquarters where you will find more of Maro’s soldiers tearing the place apart. This quest will then be completed.