As soon as you go down the stairs the two guards that were guarding the stairwell earlier will attack you. Once you have defeated them explore further what you find should be reported to Mudam.
Make sure you go speak with Lahren and get the scrolls translated. Doing this will give you the location of your brother.
Once you have spoken with Mudam you will be free to go rescue your brother.
Before you go make sure all your side quest's are completed since you will not be able to complete most of them after your brother is rescued.
Your brother can be found on Village map 3. Remember the boulder blocking the stairwell by the Library. Well it has been shifted just enough for you to go downstairs.
Make sure you save before going downstairs you will have to battle an Ogre. Usually what I'll do is throw a Hold spell on him and wade in swinging. Get the initiative and keep him off balance.
Once the Ogre is defeated step around the corner and you will see your brother.
Corvus will tell you that General Ovoron is a traitor. That he led his men to a contrived ambush and a massacre.
Feed Corvus the Herb if you have it, if not don't worry Corvus will make it back to the Citadel.
When you go up the stairs the game should transport you back to the Outer Keep first floor. Sir Roth will be there to meet you. Sir Roth will congratulate you on a well accomplished mission. He will tell you to take your brother to Brother Crosby in the Infirmary, then report to Mudam, and then go get some rest.
There is no way to rest in the game! All you have to do is purchase chapter two. Download, install, and update. Then start chapter two.