Breaching Security

Walkthrough
Forcing the Emperor to visit Skyrim, to look into the assassination of his cousin Vittoria Vici during her wedding is only the first part of Amaund Motierre's plan to have the Dark Brotherhood assassinate the Emperor himself.

As Astrid explains the weakening of the security surrounding Emperor's visit is next on the agenda. For the specifics she sends the Dragonborn to speak to Gabriella.

Speak with Gabriella
It turns out that Commander Maro will be in charge of the Emperor's security in Skyrim, but killing him outright would only warn off the Emperor, and the commander would simply be replaced by someone else. The plan is therefore to significantly distract the commander by killing his son Gaius Maro (a security scout for all the locations the Emperor will visit), and to incriminate him by planting a letter on his body. The faked letter details the Gaius' involvement in a plot to assassinate the Emperor. When Gaius is then found out, it will give the Emperor a false sense of security. All this will certainly cause a scandal for Commander Maro and his family, sufficiently detracting him, weakening security.

Gabriella gives the Incriminating Letter to the Dragonborn, and mentions that framing Gaius in a major city will yield an additional bonus.

Again, talking to the Dark Brotherhood members before leaving on the mission can be instructive.

Steal Gaius Maro's travel schedule
Off to the Penitus Oculatus Outpost it is. There the Dragonborn witnesses as Commander Maro gives Gaius Maro the special task of scouting out all the places the Emperor will visit. Wishing him all the best of luck.

As Gariella pointed out, a detailed travel schedule can be picked up from inside of the Penitus Oculatus Outpost. The important information is Gaius' first station in Solitude near the Emperor's Tower there.

Kill Gaius Maro
Scouting ahead, to then watch where exactly Gaius Maro turns up and stands around in the cities should help with the planning of the assassination.

Gaius Maro will stand in the main castle yard in front of the entrance of the Emperor's Tower in Solitude for some time. Trouble are the guards nearby.

Tip: Fall back towards the Temple of the Divines, still standing in the yard but out of direct sight of the guards. Use a strong bow and powerful arrows, sneak and one-shot-kill Gaius. This will bring the guards rushing to the target, use invisibility potions to stay out of sight.

Plant the Incriminating Letter on Gaius Maro's body
Now this is the tricky part, if there are guards around planting the evidence, the Incriminating Letter, on Gaius Maro's body will be noted by the guards (since any interacting while invisible will make the Dragonborn visible again).

Tip: Before you attack Gaius, sneak up on him, and plant the evidence in this inventory while he is still alive. And only then fall back, sneak, and kill him. Escaping now is very much easier without getting detected.

Report back to Gabriella
Return to Gabriella at the Dark Brotherhood Sanctuary and report on the success of the mission. She will be impressed and unhand the reward of, and should Gaius have died inside city limits, she will also give Olava's Token to the Dragonborn as a bonus.

If the Dragonborn chooses to give this token to Olava the Feeble in Whiterun, his future will be told.

Überwindung der Sicherheitsmaßnahmen