after a certain amount of quests you can go see the headmaster and he will notify you to go to the back of the waterfall by rainbow bridge in the commons. once there look straight ahead and there will be mortis.
there is a quest to get there the headmaster gives you a key to it you might have to do a few quests first
Uhhhh Ambrose doesn't give you the key, you pick it up after you defeat the Wraith and THEN you get the key to go back to the Death Scholl behind that door and THEN you can talk to Mortis! (To defeat the Wraith, who I forget his name, you need to have defeated the following, Triton Avenu's main boss, Alicane Swiftarrow in Firecat Alley, and last but not least... The main boss in Cyclops Lane which is General Akilles or Eyus Maximus). Hope that helps! If you need ay help just ask me! (For Wizard City ONLY) Oh, and you have to defeat somewhere around 2 or 3 bosses to get to the Death School! Their names are, Foulgaze, I forget his name but he's a Wraith (See paragraph 2 ), and someone else (if not just 2 bosses).
Sometimes quests are given before you have access to the areas in which those quests need to be carried out. If so, relax, enjoy the game, and remember than soon you will be able to go there.
Why rush? Lots of other fun things to do.
Well maybe it's not so much the rush, but I'd like to get my level 12 death school spell at, oh I don't know, level 12, around the same time most other classes get their level 12 spells.
I have to edit this post because i had the info wrong. The scroll you are looking for after going into Nightside you get from the tower to the right of the door when you first enter. So after you get the quest to get the scroll just go to the tower that looks like the dorms for the other classes in ravenwood.
You get that said quest from head master Ambrose after a certain amount of time to find a certain object. Once you defeat that said boss you will obtain a key which after speaking to Ambrose you will get the quest to enter the area needed.