The Cure for Madness

Starting the quest
Speak to Astrid and find Cicero's Journal to get the password for the Dawnstar Sanctuary.

Sanctuary in Shambles
When you arrive in the Sanctuary, you will be met by Gabriella, who, though pleased you completed your previous mission, will bid you speak to Astrid immediately. Astrid informs you that Cicero went crazy and attempted to kill everyone before fleeing. She instructs you to search Cicero’s room for clues. On an end table next to his bed is The final volume of Cicero's Journal. All the previous volumes can be found on nearby elevated surfaces. When you read the final volume, it will provide you with details of the password and location for an older and unused Dark Brotherhood Sanctuary near Dawnstar. Your next objective is to let Astrid know of your findings, and upon doing so, she will instruct you to kill Cicero and provide you with her horse, Shadowmere, as a means of getting to him. As Shadowmere is unique in appearance, you may choose to obtain him, but it is not essential to complete the quest.

Track Cicero
Travel to Dawnstar and talk to Arnbjorn who is sitting injured on the ground just outside of the Dawnstar Sanctuary. He informs you his injuries were from a fight with Cicero, but that he injured Cicero equally as seriously and you should just "follow the blood" to find him. After suggesting that he return to the home Sanctuary, enter Dawnstar Sanctuary and follow the linear path (fighting the guardians, and a troll, whilst avoiding traps) to Cicero.

Confronting Cicero
Navigating through to the end of the Sanctuary, you eventually find Cicero lying on the ground faking to be injured. He asks that you spare his life and lie to Astrid about killing him. Should the player decide to let him live, there is no effect on the outcome of the quest nor the attitudes of the other Brotherhood members toward the player. If you kill him, you can loot his body for his outfit (the jester outfit on the table at the entrance is less powerful), some gold, and leveled dagger, and you do not have to lie to Astrid. If you decide to attack him, however, he will get up and fight back revealing that he isn't actually injured.

Completing the Quest
Return to Astrid and inform (or lie to) her about Cicero's death. She will then thank you and suggest you hold on to Shadowmere "a little longer," in addition to starting you on your new quest, Recipe for Disaster.

Bugs

 * Although you can't loot the guardians after you kill them, you can pickpocket them for Shrouded Armor pieces and weapons/arrows, if you have the Misdirection and Perfect Touch perks. If done fast enough the weapons they drop when killed can be picked up.
 * You may get stuck in an infinite loop where Astrid says "You need to hear this!" and then never says anything else. To fix, simply "wait" in position for an hour. This seems to get her back on track.
 * If you kill Cicero, loot his armor set, raise him (Conjuration) and then bring him through a new loading screen his armor will re-spawn on his body.
 * If you kill the last guardian and try to loot him, the game may freeze.
 * Sometimes after retrieving Cicero's journal, when reporting back to Astrid it will tell you, "This person is busy." Simply reload the game from entering the Sanctuary, talk to Astrid, grab the journal, and report back to her - this time it should work.
 * The door in the south-east corner of the room just before the snow covered caves can be opened. Simply jump over the door bar and you will be just outside Cicero's room.
 * Sometimes after you return to Astrid once Cicero has been killed, she will say her first line of dialogue and will be unable to be interacted with. Instead when you try to talk to her she will repeat the same line of dialogue again.
 * Occasionally when attempting to leave the Sanctuary after killing Cicero, the door will not respond to Use (Or be replaced by a wet stone wall) and you can't leave. Simply restarting the console will fix this.
 * When you arrive at the Dawnstar sanctuary, you may not be able to talk to Arnbjorn straight away, if you attack him and kill him then he might talk to you.
 * Sometimes Shadowmere does not spawn outside in the pool of water. If Shadowmere doesn't spawn, stand by the pool and open your console. Input "prid 0009CCD8" and "moveto player". This will then spawn Shadowmere in the pool.
 * Cicero may not attack you in occasions when you are attempting to kill him.
 * Time may permanently slow down when fighting the Sanctuary Guardians in the Dawnstar Sanctuary, if you have the block perk Quick Reflexes and it comes into effect while fighting them, then time will still be slowed down after the effects of the perk should have worn off. This can be solved by opening your the skills or map menu, exiting and re-entering the sanctuary, or saving then loading that saved game.