Firstly, is there a need to swap out the encrypted scroll? Is it not enough to have Petrus give you a different mission?
Secondly, that friendly necromancer idea above is giving me ideas about recruiting the chaos forces instead and marching in on Attnam at the head of an army.
9 levels is fine I think. For additional length we can have an optional dungeon, like Oree is optional. Perhaps the story can be that Petrus wants to test your worth by having you clear out this "heretic's tomb", and once you've proved yourself successful, he gives you the choice to go take on a chaotic stronghold. Perhaps the game can randomly generate from the Dark Fortress (with Golgor Dhan as the boss), the Dungeon of Pain and Pleasure (with Sherarax as the boss) or the Twisted Tower (with Ischaldirh as the boss) or something like that.
Never thought I'd become reference material. I'm honoured!
Seems you already have ideas for the tomb, and I don't know a lick about balancing, I'll just make a few suggestions and leave the final design to you.
I feel there should be at least one "tactical" level, small enough that someone with high perception and/or high intelligence with ESP should be able to see the entire floor. Casters in the back/rooms whose corridors loop around so you can't walk up to them easily, dark knights in the front/open room that lets them surround you. Basically, you need to plan your moves ahead or get bogged down in a slog-fest that'll get you killed. This is a holy site for worshippers of chaotic gods, it's well defended.
You could have Xinroch's Gravekeeper as a sub-boss instead of the third form (in the flesh) 'cus he's already all rotted away. Rather, I think Xinroch himself would work better as a 2-stage fight, his skeletal form then his ghost form.
Little food! Following the theme, most enemies should be necromancers/undead/dark knights, no random animals. If you're not a follower of Mellis, you better hope one of two random rooms spawn; the kitchen or the training cage with bear(s) in it.
Similarly, since it's a tomb and not a mine, I feel it should be shorter than GC. To compensate for reduced loot, more chests spawn. Perhaps chance of a single storage room with numerous chests?