In my opinion, no modification is needed beyond sensible strategies by the lords of Rappan Athuk - who, by the way, know the maze inside out while the players don't.
As I said on the Necro boards, a large army of assailants would
immediately attract the attention of all bandit groups in the area, not to mention the spies working for the Master. This would also fundamentally alter the way the adventure runs: the original premise (as in most such adventures) is that the opponents are mostly unaware and the player characters take the battle to them.
Not in this case. The Orcus clergy would react with all of their might. They wouldn't even think twice about sealing certain passages to block advancing foes for a while. Also, the low levels would send massive amounts of troops from below, through the transporter from Tomb of Abysthor (if I recall correctly and such a device exists...) and from big Goathead's home plane.
Also unlike other dungeon expeditions, the party may not be able to return and rest. Imagine if they find that they have just descended to level 2, killed everyone and realized that on the way back, the crushing ceiling of the Mausoleum is
down , and stays that way. Naturally, the dirt tunnels from level II are also out of the question - ridiculously easy to destroy. They must now find an alternate route... No teleportation at 4th level! Thus, the Well could be a natural way (level 3, river, level 3A), but the Well is under the command of General Zelkor, an unfailing servant of Orcus! The Well also has two supply routes to and from lower levels - 7A with Phase Minotaurs, 6A with several kinds of Spiders and the surface... If the Graveyard is not secured, the party may find
themselves on the defensive, where the enemy has access to more minions from the surface!
Of course, not all is lost, since countering an invasion force will mean a large degree of mobilization. This upsets the power structure of a Chaotic Evil organization - several sub-factions may find this upheaval a great opportunit to shove foes in harm's way, use sabotage to let them destroyed, raid on their own (since there will be no time to investigate in the case of great turmoil) or even provide the party with subtle hints and aid! If the characters are lucky, don't let themselves be trapped and always keep one step ahead of foes, they can cause enough havoc to accomplish whatever goals they have and escape. If they stay and fight, they will fare like Bofred's group - they will be slowly ground up and annihilated, since the Master has more resources and, knows the dungeon's layout [there are several connections which could be used for complicated, elaborate supply routes and ways to set up ambushes].
That's how I would do things.
