Head towards the newly unlocked east door (the above screen) to start exploring another section of the tomb.
The first major battle will commence as soon as you enter a larger chamber. First off, take care of Undead Prowlers, and don't forget to dodge often and in the right moment. Further on, the battlefield will also welcome Skeletal Champions. In this case I recommend the trusted technique of using Death Grip. Sadly, defeating them all won't end the mater - a new boss awaits you.