King Lysandus' tomb is a complex maze, and finding the King's sarcophagus is not an easy task. After entering the tomb, go through the door on the left and follow the left wall until you see a floating skull. Click on the skull and you'll be transported to a torture chamber further on within the tomb. From this point you'll need to search the tomb carefully until you find an elevator that takes you down to a lower level. Continue searching until you find another elevator down, go down into a small area where you will find a lever on the floor and pull it. Go back up the lift, out the door and turn right. Go to the end of the corridor and open the door, cross the huge chamber and at the back of the chamber on the left side is a column with an opening in it (you just opened the column by pulling the lever on the floor). The central area of the chamber will now sink down, revealing the sarcophagus of the King below. (I'll attempt to describe this in more detail at a later time, the instructions for locating the King's body are complex).
After you've located the sarcophagus, just click your cursor on it. An animated cut-scene will follow in which the King tells you the name of his murderer and asks you to avenge his death. He will tell you that Lord Woodborne is his slayer and his stronghold will be marked on your map. For an excellent set of instructions for finding King Lysandus' corpse, go to the Daggerweb Walkthrough.