A Kiss Before Dying
Oblivion: Dark Brotherhood Quest
These orders are found in the Ayleid ruins of Nornal. Follow the path inside and you'll find an underwater gate. You can pick the lock, or search the ruins until you find its "rusty key". Even more easily, though, you can jump out of the water and between the bars above the gate. The chest on the other side contains your orders.
Your target is Ungolim, in Bravil. You'll find him visiting the Lucky Old Lady statue in the city every evening (6:00 pm to 1:00 am). Ungolim will attack you immediately.
He carries an extremely powerful bow which will knock you off your feet. You can solicit the guards' help by hiding behind them if you need to.
This quest is completed with Ungolim's death, and you'll receive your next quest, Following A Lead immediately afterwards.