The Cure for Madness

Walkthrough
After successfully finishing Breaching Security, not all is well at the Dark Brotherhood Sanctuary, it seems Cicero's sanity finally left.

Talk to Astrid
Joining up with Astrid, it becomes clear that Cicero suddenly attacked and badly wounded Veezara who was protecting Astrid. Cicero then ran off leaving the Sanctuary for places unknown, followed by the infuriated Arnbjorn out for blood after the attempted attack on his wife Astrid.

After talking to the other members of the Dark Brotherhood, it seems that dismissive comments about the Night Mother in the presence of her Keeper Cicero must have finally made him snap.

Search Cicero's room
The only hope to find out where Cicero stormed off to is to search his room thoroughly.

Read Cicero's diary
And indeed Cicero's Journal - Final Volume on the end table next to his bead reveals that Cicero knew of and had visited another Sanctuary in Skyrim, and was able to find the password to open up the door to it as well.

Talk to Astrid
Passing along the information it is clear that the Dragonborn must make haste and travel to the Dawnstar Sanctuary, a place not used in over one hundred years.

Behold Shadowmere
And to speed things up, Astrid suggests using a very special horse, that it pretty much "one of them". Outside the door of the Sanctuary awaits Shadowmere. The Dragonborn mounts the mysterious horse and rides off to Dawnstar.

Note: If you already discovered Dawnstar and the Sanctuary there, you need not ride there on horseback using roads and path, use fast travel instead. The Dragonborn and Shadowmere will get there together.

Locate Arnbjorn
In front of the ominous black door to the Dawnstar Sanctuary the heavily wounded Arnbjorn will be catching his breath.

Talk to Arnbjorn
He mentions that he did wound Cicero, so it should be easy to follow his trail of blood. He could not follow Cicero past the door though, missing the password.

Enter the Dawnstar Sanctuary
Having read Cicero's journal the Dragonborn knows the password though:
 * "Innocence, my brother."

So entering the Sanctuary is not a problem.

Kill Cicero
Passing through the Sanctuary Cicero will call out to the Dragonborn, reflecting Cicero's increasingly distressed and weakening state. To find the way through the Sanctuary following the blood trail helps. Fighting off the total of twelve Sanctuary Guardians is the biggest issue.

Kill Cicero, or leave the Sanctuary
Once the Dragonborn finally comes face to face with Cicero, remaining silent opens up the option to not kill Cicero, who after all, only tried to defend the Night Mother's honour as her Keeper. Not having killed anyone in the process.

So it is up to the Dragonborn to decide what to do.

Report back to Astrid
Leaving the Dawnstar Sanctuary, return to the Dark Brotherhood Sanctuary and talk to Astrid. Should Cicero still be alive, the Dragonborn will lie about his death.

Astrid explains the next step in the plan to assassinate the Emperor is on the way, and that this time Festus Krex will take on the mission briefing.

Der Wahnsinn hat ein Ende