Some redditors are going to call me crazy... perhaps a little mad. But not you dear Listener, you're going to believe me... and let me live. He he heee!
I discovered this after I had learned that the Amaund Motierre you meet in The Bannered Mare has a different model to the one you met in Volunruud. I was trying to figure out why it could actually be a different person as I played through the Dark Brotherhood questline. Amaund sent an imposter in his stead? Killed and replaced for reasons I'd have to figure out?
I was on my way from the Falkreath Sanctuary to Dawnstar during The Cure for Madness and I read Cicero's journals. When I finished I was like, "Hang on, this is a "descent into madness," it's a great way to disguise somebody being replaced by somebody else." I knew that wasn't enough though. If I made the claim based on that evidence alone I would be mad.
I got into the Dawnstar Sanctuary and it had been a long time since I'd done the questline. I looked on the first table and what is sitting there? The Jester's Clothes! Not only that, they have the same enchantments as Cicero's Outfit. Why does the Jester have assassin's enchantments? He's the assassin.
What happens during this so-called descent into madness is the Jester kills Cicero. He then disguises himself as Cicero and infiltrates the Cheydinhal. He gets one member to kill another, another dies and the other one disappears. Once he's satisfied the sanctuary has been destroyed he can then stop pretending and return to his real personality:
21st of Sun's Dusk, 4E 192
Cicero is dead! Cicero is born!
He then moves on to destroy other sanctuaries - namely Skyrim. He is the "someone on the inside" who gives the pass phrase to Commander Maro if you see him after killing Astrid. It's widely assumed that it's Astrid who gives the pass phrase to Maro but she only contacts him deep into the storyline. Her goal is just to betray you and not the sanctuary.
It becomes much clearer when you understand why there is a different Amaund Motirre in The Bannered Mare, but that dear Listener is another essay entirely.