The time has come to kill the Emperor. Go talk to Astrid who will give you the honor of being the Gourmet. She’ll also give you some Jarrin Root, an ingredient that will instantly kill those who eat it. Yipes! The goal is to pose as the Gourmet and make the Emperor’s meal, putting that in it.
Head over to Solitude where Commander Maro will be guarding the tower. Show him the writ and he’ll be surprised that YOU are the Gourmet, but will let you in, telling you to talk to Gianna. Head inside and before chatting with Gianna loot the place. Especially upstairs, where you can find an ENCHANTING SKILLBOOK called the " Catalogue of Weapon Enchantments " near a locked door.
Go down now and talk to Gianna. She’ll make you put on a chef’s hat before cooking, but once you talk to her again you can start picking out the ingredients to put into your famous dish. Pick whatever you want (I recommend the weird ingredients as the dialog here is hilarious!) and then follow her as she goes to serve it to the Emperor.
Once she unlocks the door to the Emperor, quickly loot the room as best as you can then get over to the opposite door (across from the way you came in). The Emperor will applaud you on the soup before dying, and then the game will tell you to go through that door, but since you’re there you can do it asap.
Well, it looks like we’re home free! Go forwards and… damn. Maro advises you that a fellow Dark Brotherhood member gave you up to save their Sanctuary, but he’s not only going to kill us he’s also going to slaughter the Sanctuary. You’ll have to fight three agents now (or you could run past them and run away to Skyrim proper down the stairs). No matter what you do though, you now have a 1500 Gold bounty in Haafingar. Damn.
Note: Even if you kill/escape from the guards, when you get back out to Skyrim you may be unable to fast travel because "a guard is chasing you". Even if you wait, no guard may come. I got around this by finding a guard (near Katla’s Farm ) and killing him. Just FYI.
Our goal now is to get to the Sanctuary. You’ll be unable to fast travel to the destination, so fast travel to Falkreath instead and head to the Sanctuary to the west. As you approach, you’ll have to kill off four more agents who are guarding the outside. Kill them off and approach the Black Door to finish this quest and start the next one: "Death Incarnate". Oh, also note the corpse on the tree nearby. One brother gone…