Damn Jester!

WARNING:SPOILERS

The Dark Brotherhood is a shadowy organization of secret assassins, said to be disbanded from a group based in Morrowind known as the Morag Tong. Although the missions they conduct aren't as "legal" as their older cousins, they can carry out a contract just as discreetly and have just as much power and influence. Instead of receiving writs of assassination like the Morag Tong, agents of the Dark Brotherhood are summoned by performing the Black Sacrament, a ritual in which an effigy of the victim is created using human bones, flesh and organs rubbed with the nightshade flower and stabbing it repeatedly while reciting the following:  "Sweet Mother, sweet Mother, send your child unto me, for the sins of the unworthy must be baptized in blood and fear." This plea then goes through their prime entity, the Night Mother, who passes the message on to the Listener, who in turn continues the chain to the collective. Their leader is a female Nord by the name of Astrid, a Redguard named Nazir being the second in command. Their headquarters is located in their Falkreath Sanctuary, later moving to Dawnstar.

While in the snowy city of Windhelm, you may overhear guards or other passersby mention a young boy holed up in his house, performing something called the Black Sacrament. Having piqued your interest, you decide to do a little B&E into the residence of Aventus Arentino. Upstairs, you will find him with all the fixings for the ritual and will assign you the contract, yourself posing as the assassin summoned. He will tell you about his escape from Honorhall Orphanage in Riften and insists that the old crone running it, Grelod the Kind, is anything but. Travel to Windhelm, kill the witch, and return for your reward. Rent a room in Candlehearth Hall and rest. But do not do so lightly, for you will surely be in for a surprise when you wake.

Upon awakening, you will find yourself in a small shack you will later discover is outside the city of Morthal. A female, revealing herself to be Astrid, is laying on top of a bookcase telling you the contract you accepted was originally meant for her organization. In order to repay her for your actions, she will direct you towards the back of the room where there are three people kneeling with hoods over their heads and instruct you to kill one of them (note: you may instead choose to kill Astrid, opting to destroy rather than join the Dark Brotherhood). She will then give you the key to leave the shack and invite you to their sanctuary near Falkreath. It is there that you will be officially inducted into the shadows and given your first set of assassination contracts. Complete them and report back for your next assignment.

You may notice something amiss in the main sanctuary area; specifically a man dressed as a jester (the Keeper, Cicero) with a large box containing the Night Mother. Astrid welcomes him in with reluctance and gives you your first real assignment as an agent of Sithis. You are to travel to Markarth and speak with a woman named Muiri about an ex-lover she wants taken care of, a bandit leader hiding in the Dwarven ruin of Raldbthar named Alain Dufont. Kill him, suggestedly using the crossbow traps on the ledge above him, and collect your reward from Muiri and Astrid.

Astrid will tell you that she senses something wrong with Cicero, who's been locking himself in his room at night and talking to himself. Hide in the Night Mother's coffin and listen to what Cicero says, apparently having been talking to the Night Mother all the time before. But something happens that's not anticipated: the Night Mother will start to glow and speak to you, informing you that you are the next Listener and to pass on a call made by Amaund Motierre in the ruin of Volunruud. Cicero soon discovers you and calls you a traitor, until he learns of the Night Mother's declaration of the Listener, information that causes him to dance jauntily in place. Inform Astrid of the Mother's given contract, who will tell you to receive new contracts from Nazir while she thinks it over.

When you return for your rewards from the preceding contracts, Astrid will approach with her decision. Follow the Night Mother's orders and go to Volunruud, where you will receive a very intriguing contract; no doubt a milestone in their history and just what they'd need to gain much needed influence and fear: assassinate the Emperor of Tamriel. He will hand you a jeweled amulet to cover expenses and a letter to Astrid. Return to the sanctuary and give Astrid the letter. She will read it and tell you to get the amulet appraised by an old friend in the Thieve's Guild, a fellow by the name of Delvin Mallory. Find him in the Ragged Flagon and give him the amulet. He will identify it as belonging to a member of the Elder Council and will comment on your "friends in all sorts of places." He'll give you letter of credit in exchange for the amulet, as per his and Astrid's "standard arrangement." After receiving the letter of credit, you will be given your first task in the assassination of the Emperor: kill a cousin of the Emperor on her wedding day.

Before taking on this special occasion, it is recommended to talk to other members as they give rather good strategies for eliminating the target. If you seek the seemingly young alchemy master Babette, she will suggest dropping the statue hanging above where she will be standing on the balcony. If you consult with Gabriella beforehand, she will tell of a bow, a few arrows and an archery potion on a parapet with a clear shot for a long-range kill. Whichever method you choose, kill her and get the hell outta dodge with Veezara covering your escape. Your next task in the intricate weaving of the plot to kill the Emperor is espionage work. Kill the commander of the guard's son Gaius Maro and plant a false letter on the body, placing him in a plot against the Emperor. While everything seems to be going according to plan on the assassination attempt, something's gone wrong among the group.

Cicero has apparently made an attempt on the lives of many members, including herself and the now grievously injured Veezara. Check his room for clues to where he went, the final journal revealing he has hidden away in the sanctuary outside of Dawnstar. Take up Astrid's horse Shadowmere and ride off to the sanctuary. Outside, you will find an injured Arnbjorn saying they injured each other and to "follow the blood."
Fight your way through the ghosts of former members and past a unique troll, the Undefrykte, towards Cicero. You will find him in the final room in a fetal position, begging you not to kill him. If you choose to, he reveals he is indeed uninjured and will fight back. Having dealt with the jester, continue with the contract.

The next step to this mission is to kill and impersonate the Gourmet, the highest up on the Emperor's kitchen staff, and poison the Emperor's food with the highly lethal jarrin root. Obtain the identity of the Gourmet, kill him, hide the body, and take up his role. Present the note looted from his body to Commander Maro to gain entrance to Castle Dour in Solitude. Assist the other chef in conjuring the concoction of death and watch as the two of you bring the Emeperor his last meal. Sneak out of the room as he tastes it and cross the bridge where a surprise in the form of information is awaiting you. Commander Maro appears with a tale of betrayal. on both parties. As it turns out, someone inside the organization had informed them of their plan. So they used a decoy and let them carry on with the operation in exchange for your life. The other betrayal? Maro intends to wipe out their organization as revenge for killing his son.

Arrive at the sanctuary to find several of the Emperor's guards outside and Festus pinned to a tree by the entrance by dozens of arrows. Enter the sanctuary and move past the bodies of Veezara and Gabriella, past a werewolf Arnbjorn fighting a losing fight, all the way to Nazir fighting a Penitus Oculatus Agent named Arcturus. Assist him in killing the agent and talk to him. Find an escape route, during which the Night Mother will tell you to hide in her coffin for protection. Several explosions and rumbles later, Nazir will open the coffin with Babette standing next to him, telling you Astrid is still alive and wishes to see you. She can be found in an alcove inside of her old room. She is found laying on the floor, body completely burnt with a circle of candles around her, having performed the Black Sacrament on herself. She reveals that she is the traitor for she didn't yet believe in the Night Mother and was jealous of your being named Listener. Take up her blade and deliver her will of the mercy killing. The organization still requiring it, you must continue the original job and finish what was started.

Travel to Whiterun and meet with Motierre on his ground floor room in the The Bannered Mare. According to Amaund, the Emperor is aboard is ship, The Katariah, docked in the Solitude inlet. He will also inform you that Commander Maro is on-board if you would like to exact revenge. Sneak aboard the ship, kill Maro if you wish, and tiptoe your way past several PO agentsto the Emperor's quarters. The Emperor will greet you immediately after entering, having been expecting you knowing it is his destiny to die by your hand. However, he will request one last thing before you kill him; he wishes that you kill the one who orchestrated this attempt as he may be doing it for personal gains. Kill the Emperor and meet back in Amaund's room. His reward is revealed to be in a dead drop in the room where you first met him. If you so desire, you may fulfill the Emperor's last wish and kill him. Regardless, collect your reward of 20,000 septims and return to Nazir, now based in the Dawnstar Sanctuary. The Dark Brotherhood has now gained influence and fear over many once more, thanks to you.