After talking with Astrid, follow her instructions and find Gabriella within the Dark Brotherhood Sanctuary. She will explain that the Emperer's security force, the Penitus Oculatus, will begin its preparations to protect the Emperor. To pull this off, Gabriella and Astrid, have a plan. They plan to break Commander Maro by having you go and kill Gauis Maro and plant some false evidence implying he has a plot to kill the Emperor.
The bonus is earned in this mission by killing Gauis Maro in one of the major cities that he'll be visiting. Astrid will reward you with a token to give to Olava the Feeble. This will let you engage is a side quest that is otherwise unavailable: Locate the Assassin of Old.
No matter which path you plan to take, you will need to head to Dragon Bridge. Once there you will find the
To steal Gauis Maro's Schedule, head over to the Dragon Bridge. Go inside the Penitus Oculatus Outpost that sits just behind them in Dragon Bridge. Sneak inside at night and you will only have to deal with one Guard. You can easily just sneak in and he will, likely, ignore you. Go over to the table and read note there. Steal it and then exit the Outpost.
You will want to wait about Whiterun until Turdas evening. This will put Maro in the Bannered Mare that evening. Wait until midnight that evening. This will have him appear and go to sleep in the room on the ground floor, opposite the entrance. Once he goes to sleep, slip into the room after him. This will let you easily dispatch him without any witnesses. You can loot the body for the Penitus Oculatus Armor Set. After that, plant the Incriminating Letter.
Simply slip away after you have planted the letter. No one is likely to know about your deed, but sneak out of the room all the same. Just leave the tavern and then make your way back to the Dark Brotherhood Sanctuary.