Bound Until Death

You must journey to the Temple of the Divines in Solitude, attend the public wedding reception of Vittoria Vici and Asgeir Snow-Shod, and kill the bride. You will receive a bonus if you kill Vici as she addresses the crowd.

Entering the temple
Travel to Solitude, then walk over to Temple of Divines. In front of the temple there'll be a group of people gathered. In two chairs standing opposite of the temple door the bride and groom will be sitting. Talk to the bride to start the event.

After you have spoken to her, you may enter the temple. Take the first stairs on the right, walk upstairs and then out the door which should be on your right. You will now end up on top of the roof. If you look down to your right, you will see the crowd you were standing in the middle of only a minute ago. Walk towards the gargoyle standing over the balcony.

The Balcony
After about thirty seconds, the bride and groom will get up from their thrones, and enter a door. This door will lead them to the balcony above their place of honor that faces the assembled guests.

Sniper
Gabriella tells you that she has taken the liberty of scouting the area and even leaves you Firiniel's End an enchanted elven bow that does 20 frost damage, Elven Arrows, and a potion of true shot on the balcony across from where the bride delivers her speech. You have a clear shot from this point but there will be a chance that you are discovered immediately after the shot and be pursued by the guards. To get around this, have some sneak increasing enchanted boots or a high sneak skill. Also a perk to this is you can immediately fast travel the Dark Brotherhood Sanctuary.

Statue
Speaking with Babette shows that a gargoyle statue above the balcony where Vittoria gives her speech is loose. Unhinge the statue during her speech to gain a bonus. It is possible to drop the statue on her while standing next to her on the speech platform by crouch hopping. This will make it easier to quickly loot her body but requires high sneak skill/apparel. The wall section / walkway above the bride gives you access to the gargoyle as well. When the bride starts speaking, pushing the loose gargoyle while in sneak, preferably, gives you the best escape route (done right and with the proper sneak skills, no one will even see you), as well as give you the bonus for killing her in the middle of her speech. Note that all the guests and the groom will now be hostile. However, you will not be given a bounty. There is a window between when you hear the crash and when someone calls out where you can fast travel out, making your escape easy.

Plain, Old-Fashioned Murder
If you wish to make a daring escape with the guards hot on your tail, you may simply waltz up to the balcony with the happy newlyweds and plant your knife in her throat, or fire an arrow into her heart from the courtyard filled with spectators. This method does incur the standard thousand gold bounty, just like any other murder, but you will still accomplish your bonus objective, and you may use the thousand gold payment from the contract to pay off your fine. Apparently there will indeed be a bounty when you slay the bride, following up behind them, on the balcony (sneakmode of course), but when you slay the husband, the only witness dies, and the bounty is reversed. With a bow, it is easy to snipe the bride while standing on the ground behind the crowd near the door of the temple, next to a small boy. After sniping, immediately turn around and go through the temple doors. The crowd will not follow you and you can fast-travel at your leisure.

Werewolf
If you complete The Companions quest to the point where you become a werewolf, with a sneak skill, you can hide above the balcony where you simply transform, jump down and kill her. If done right, you will not gain a bounty and you will receive the bonus. If your sneaking skill is not the best, you can still hide above the balcony but on the opposite side there is a nice little colve. (I find this a useful way to kill any target in a city and not gain a bounty.)

No Blood on Your Hands
If you'd like to play Skyrim being a "good" person (i.e. not killing anyone), and have the Invisibility spell (or potion), and also have any Fury spell (Fury, Frenzy, or Mayhem), then you can take this approach, which can only net you a bounty of 40 at the max. Simply go to the balcony on which you look down and to the left to see the bride (this balcony/bridge is higher than the place that the bride speaks from) and prepare. While waiting, you can prepare by pulling out the two aforementioned spells and sneaking. When the bride begins her speech, shoot the Frenzy spell at her, and go invisible, using your preferred method. Back off quickly into any place where you cannot see the wedding (and therefore, the guests cannot see you) The husband will kill the bride, and the objective marker should be changed to Report Back to Astrid. Wait until you are hidden, and fast travel. Sometimes this glitches, and can cause you to lose your bonus, however.

Escaping
If you don't escape the area immediately (easily done from the wall above the gargoyle), Argonian Veezara will now appear and tell you that Astrid sent him to assist you. He will take care of the crowd, allowing you to slip away. You should not attempt to fight the crowd, as killing some of them will give you a bounty. The guards won't assist you nor attack you. Travel back to Astrid to finish the quest.

Achieving the Bonus
As long as the bride dies during her speech on the balcony, you will receive your bonus, regardless of the method used.

It is highly recommended to get the bonus as it acts as a follower entirely.

the Bonus is Unique because it is Lucien Lachance, A Speaker for the Dark Brotherhood in oblivion

Bugs

 * It is possible that, no matter how you kill Vittoria, you will not receive the bonus.
 * Reloading can sometimes solve this, waiting until the middle of her speech.
 * This most often happens if you fast travel in the time gap between the gargoyle landing on Vittoria and a guest screaming about murder. Six consecutive attempts and reloads yielded no bonus from fast-travelling, but waiting for Veezara to mount a distraction, fleeing Solitude and taking a carriage to Falkreath easily netted the bonus.
 * After killing Vitoria, you can not fast travel even after running for a long distance.
 * It is possible that when killing the bride it says you have 1000 bounty but it is not true. BUG: Bounty was still there after fast travelling from Solitude and back again.
 * Despite the killing occurring in Solitude, you can obtain a bounty of 1000 in The Rift.
 * There is a bug that causes Vittoria to disappear from the balcony as soon as you enter the temple to find the gargoyle. She will appear back on the throne next to her husband.
 * There are instances where after the public killing, your follower is involved in fighting off huge numbers of guards while you are trying to run away. After fast traveling and successfully escaping, you may find your follower missing, meaning the follower may be dead. One way to prevent this is to send your follower away before killing the bride. Then after successfully escaping fast travel back to the first meeting location of the follower and recruit them again.
 * If you have the problem of Dragons appearing in cities(even fortified ones), when you knock over the Gargoyle no one will even notice you killed Victoria because the dragon will take all the attention. Just kill the dragon to fast travel.
 * Sometimes her arms bend back.
 * It is possible that upon pushing the statue onto Vittoria, even while hidden and with a high sneak skill, both the crowd, guards and all other residents of Solitude will become hostile or flee. A possible workaround for this is to simply exit the city without drawing your weapon and to carefully jump down the cliff by the warehouse, then into the water. Swimming to the other side negates any hostiles in Solitude and you can then fast travel.

Note

 * If you loot the bride after you slay her, you can receive the following: Vittoria's Wedding Band, Wedding Dress, Wedding Sandals & Wedding Wreath.
 * Sometimes you can just yield to the groom and the reception will cease their attack. Veezara will still come to aid you (useful if you happened to incur a bounty for the murder).
 * One thing that seems to work to escape is right after pushing the activate button to push the loose gargoyal is to take a Potion of Brief Invisibility.
 * Even if you were never detected and Veezara didn't show up, he will later (when talking at the sanctuary) tell you that it was "a pleasure to fight by your side".