This is the start of a long task of killing the Emperor of Tamriel. In the first part you have to go to Volunruud, where you can find a word of power, a full guide is here, and receive the task from Amound Motierre. Next you have to go to the Thieves Guild and speak to Delvin Mallory who will give you a letter of credit. I'm not sure what happense to it, but you never see any of the money from it as only Astrid can use it to obtain an item from Delvin Mallory.
