Skyrim:Breaching Security

Quick Walkthrough
#Speak with Gabriella in the Dark Brotherhood Sanctuary.
#Travel to Dragon Bridge.
#(Optional) Steal Gaius Maro's travel schedule.
#Kill Gaius Maro and plant the incriminating letter. For a bonus, kill him in a major city.

Detailed Walkthrough
Sins of the Father
When you return from your fateful visit to Vittoria Vici's wedding, you will find a gleeful Astrid ready to move onto the next phase of the plan. She instructs you to visit with Gabriella for the next mission. Gabriella informs you that with the Emperor traveling to Skyrim, the Penitus Oculatus will be securing the way. In order to disrupt the security surrounding the Emperor, you will need to cripple its head, Commander Maro. The Brotherhood's plan to accomplish this is to kill Gaius Maro, the commander's son, and plant a letter implicating him in a plot against the Emperor.

Dragon Bridge
Gabriella has instructions on how to locate Gaius Maro, as well as how to earn a special bonus for this contract. To begin, travel to the town of Dragon Bridge and, if you choose, steal Gaius' travel schedule. This makes things much easier, and you really have no reason not to do it. Additionally, killing Gaius in any city other than Dragon Bridge will grant you the "reading of your future", according to Gabriella, but this is optional. To obtain the schedule, simply sneak into the building where the schedule is located and nab it. Even if you are detected, with weapons sheathed and without casualties, a soldier will only yell, ''"Hey, you won't get away with that!"'', one bounty is incurred and they do follow you. (To avoid a bounty, wait until the door shows as being unlocked to go inside. Then, once inside, look to see how many guards are inside. Usually there is only one awake and walking around. Wait for him to go outside and you can now take Gaius Maro's Schedule with no bounty, because the other guard(s) are asleep. Just to be safe, you can always sneak to take it.) Once outside, check what day of the week it is in-game, then read the schedule and travel to the appropriate city.
This is the schedule as it appears in game that you steal from the Penitus Oculatus Outpost:



The Stalker
Don't be afraid to simply wait for a day and follow Gaius around. There is no time limit on this quest, and when Gaius has completed his schedule, he will repeat the same schedule next week. It is much easier to assassinate him on the road when he is alone, but you forfeit your bonus reward by doing so. Wherever you kill him, loot his corpse and plant the incriminating letter on his body, then return to the Sanctuary. Alternatively, you can reverse-pickpocket the note prior to killing him.
Some strategies for completing the contract are to wait until he stays at the Bannered Mare in Whiterun. He will go to sleep in a room alone around midnight, allowing for an easy sneak attack. He might also be alone while he's eating in the Windhelm barracks. Conversely, his notes suggest that he doesn't actually sleep in any of the inns or the barracks in Riften, and seems to spend most of his time in the main hall with plenty of watchful eyes, so it might be wiser not to bother him there.  Alternatively, you can speak to Gaius and provoke him into attacking you; this allows you to kill him in self-defense without a crime being registered.

General Strategies
Solitude
Should it be Morndas as per Gaius' schedule seen in the Penitus Oculatus Outpost, he will be in Solitude overlooking Castle Dour's courtyard and the Emperor's Tower during the day. By evening, he will retire to the barracks below where General Tullius and Legate Rikke are normally found. This is a good spot to attempt to stealth kill him as he normally sits at the table just around the corner from the stairs. The guards will be asleep, and thus unaware of any attempt on Gaius, unless it is around 8:00 AM or 8:00 PM. Both these times are when the changing of the guard normally occurs, and the area will be full of guards retiring for some sleep while their relief gets up to head out on patrol. This is especially so if Captain Aldis is himself resting for the night, in which case he might see the murder and trigger a bounty. The chances of this happening are higher if sitting with Gaius when he is killed. A bow or crossbow may be the best option here by hiding in the shadows near the barrels in the kitchen area.

A good opening is when Aldis stands up to walk to the far end of the beds. With his back turned, an opening is made to stealth kill Gaius. Even if Aldis or the guards hear it, with a high enough Sneak, you will generally remain undetected. However, if Gaius is spoken to and made hostile, the guards will awaken and come to your aid, since they'll consider Gaius the criminal for attacking you first. In this case, you will need to deliver the final blow before Gaius becomes non-hostile again, risking a bounty should he not be killed and regains his health. Alternatively, the Ebony Mail acquired from Boethiah's Calling can be used to suck his health away with its poison enchantment.

Riften
In this particular city, if Gaius is there to do his rounds on Middas, the alleyway to the left provides an opportunity (the lock on the gate may need to be picked unless the Skeleton Key has been collected). As Maro leaves (around 1:00 A.M.), sniping him with an arrow will avoid detection and any bounty. If the archery skill is at a low level and/or the player does not have the appropriate perks, he may not die with one sneak attack, so there may be a prolonged fight. Another option is to wait in the corner of Riften Castle's courtyard, behind the small grouping of trees, and when he leaves (he exits that side every time at around 11:30 A.M., although the actual times may vary), he will be alone and far enough away from any guards to allow him to be killed without any worry about a crime.

Palace of the Kings or Other Guard Barracks
Gaius will sleep in the barracks with the guards in Windhelm while there for his rounds on Tirdas. This can be one of the hardest places to get him without being detected. Around 10:00 P.M., in the corner by the sleeping guard, he can be sniped with a bow or crossbow (preferably after having coated the arrows with poison). Each time he is shot, he will get up and look around the room. He can be continually attacked without being discovered. The guards in the beds will sometimes get up and look as well, but with some luck and stealth, will ignore you, who should be in the shadows. After Maro is dead, once the guards settle down it is possible to creep over to his body and plant the Incriminating Letter. Sneaking back to the corner, then jumping over the beds with the guards in them until the doorway is reached, allows you to sneak out to freedom. Just as it is in Solitude, if Gaius is made hostile by speaking to him, the guards will awaken and support you since Gaius just made himself a criminal in their eyes by initiating an attack.

The Bannered Mare
In Whiterun when he is there on Turdas, it is extremely easy to sneak into his bedroom in The Bannered Mare, as it seems to be the only time he is not bunking in a castle or in barracks. He can be killed in his sleep with the door shut, and then the letter planted. This does not result in a bounty.

However, there is a high chance that Maro might fall behind on his itinerary schedule and therefore only make a brief stop at The Bannered Mare, for some drinks, before continuing on to Markarth. In this case he normally arrives alone on Turdas evening (10:00–11:00 P.M.) and heads straight to the hearth in the middle of the inn for a drink, whilst enjoying the bard's performances.

With a high level of Sneak and the Ebony Mail, while sneaking he can be shot using a ranged weapon from the balcony of the room overlooking the hearth (it is the same room rented from Hulda). Since this balcony is full of shadowy spots it is possible to hide in one of those and wait for Gaius Maro to charge upstairs looking for his assailant. Once he has touched the shadowy tendrils of poison emitted by the Ebony Mail, it is just a matter of waiting for him to fall lifeless. Then the Incriminating Letter can be planted and the scene left with no alarm raised.

Obviously, this technique can also be used in other dark locations as well. A word of caution though: killing him in public usually leads to collateral damage, so there will be some bodies on the way out of the inn. 

Sometimes, he can be killed in full view of guards and citizens. The guards will do their normal "What was that?" routine, but will not see him actually being slaughtered, even though they see you plainly. Citizens, however, will scold you and sometimes turn hostile when they see Gaius Maro killed.

A Direct Provocation
Another alternative method is for you to approach Maro and use the option of telling him how he will be killed, and then the Emperor. After this, Gaius will begin to attack. Letting him strike the first blow, then fighting back and killing him will not cause a bounty. This method can be used in any major city in order to receive the bonus. If using this method, it is best to confront him in view of a guard – so that it is seen that Gaius attacks first, and the guards come to your defense. However, if a guard wanders into view of the fight as it is proceeding and the first blow he sees is struck by you, a bounty will be earned.

If you are a member of the Thieves Guild and happen to encounter Maro in Riften, he can be provoked, and fellow Thieves Guild members (and even Maven Black-Briar) will come to your defense. They will finish him off. This can also be done in Whiterun, where the Companions are available, or similarly, in the College of Winterhold, where the mages will ally themselves with you, should you be part of their guild. More generally, if Maro is attacked in a town or city where friends are present, they will help to kill him. If he happens to enter your house or homestead, the Housecarls can also be immensely valuable, since they will fight him off too.

Alternatively, if he is killed anywhere in a city where the you are Thane, your position as Thane can be used to remove the bounty.

Dragon Attacks
The bonus can also be received just outside a city. If a dragon attacks just outside a city, he can be killed while the guards are fighting the dragon, resulting in no bounty. The bonus will still be counted.

Dragonsreach
An easy way to kill Gaius without good combat skills is when he is in Dragonsreach, in Whiterun. He can be approached and provoked. Once he attacks, the entire court of Whiterun will assault him. However, they will not kill him, but rather, will lower his health down to the point that he falls to one knee. This allows you to slaughter him with an extra attack or two.

Pickpocket
With a high enough Pickpocket skill, it is possible to plant the Incriminating Letter on Gaius before killing him. This allows him to be killed at range and in the open, and the quest completed without approaching his body. With the pickpocket perk Poisoned, poison can also be placed in his pocket along with the note. The stronger the poison the better.

Beast Form or Vampirism
While in Whiterun, after planting the Incriminating Letter on Gaius, he can be murdered using Beast Form or Vampire Lord form. Afterwards, escaping to The Companions' Underforge, it is possible to wait until returning to normal appearance.

Immediate Hostility
Gaius can be attacked while outside a major city but not killed, by running away and losing him. Once he has entered a major hold, he will attack you on sight. Since he attacked first, he can be killed with impunity. The guards will either help you or ignore the scene.

Markarth
At some point while in town on Fredas, Gaius will go to the guard tower in Markarth. Once he is back out of the tower, Unrelenting Force (preferably all three words) will launch him off of the high point that the guard tower is on. The player can augment this with Force Without Effort and/or Dragonborn Force to enhance its effects. After following him down, Gaius will attack once he is back on his feet, causing the guards to see this as a crime and attack him until he yields. He can then be simply finished off, and the Incriminating Letter planted.

Change of Schedule
Whenever Gaius sees you he begins taking an alternate route. This can make the quest quite hard as he will turn away from a city if he sees you anywhere near it. If this happens, waiting in a tavern in a city on his route will allow him to eventually resume his normal journey.

Reward
For this assault on the emperor's security, you will be rewarded with leveled gold. Gaius himself carries a set of rare Penitus Oculatus armor. If Gaius died in a city, you will also receive Olava's Token. The token will give you a miscellaneous quest to find an "assassin of old", which will lead you to a dead Dark Brotherhood member in a cave equipped with the Ancient Shrouded Armor set. This set is the best Dark Brotherhood armor that you can acquire. It has a higher armor rating, higher gold value, lower weight, as well as having a superior enchantment to the base Shrouded set.
 

Notes
Since Dragon Bridge is part of Haafingar, the guards there will try to arrest you if you earned a bounty in Solitude during the previous quest, which will interfere with this quest. If this is the case, travel to Solitude first to clear your bounty.

You may trigger the quest stage without paying the bounty in Solitude by sneaking or using an invisibility potion or spell. For example, get into a sneaking position and fast travel to Dragon Bridge. Then, drink the invisibility potion and sneak behind the house to your right. Stay hidden behind the house as you listen to the target’s dialogue. Wait for him to leave Dragon Bridge and fast travel away.

Bugs
 
 
 
 
 
 
 
 
 

Quest Stages