"Find Abbot Suger's tomb" is a mission objective in Stories of Franciade / Last Will
Walkthrough
Head to the Northwest from the entrance of the Royal Crypt. When you come to the Crypt itself, head to the Northwest and into the second corridor. Go to the left and then take the first door on the right. Just inside you will find the casket you are looking for. There is a problem though as the Headstone is missing.