First of all, I'm not using the Dark Heresy rules, or the Calixis sector, so no need to go into specifics, but I figure you might have some nice general suggestion anyway, so here goes ...

The situation is as follows :

A massive Chaos invasion has struck the planet where the PCs work as Inquisitor assistants. After about a dozen scenarios "out in the field" they are pulled back into the planet's main Hive City as the Chaos forces start to lay siege to it.

Now, I have some campaign-specific scenarios prepared (old enemies, betrayal from within, unstable Psykers etc.) but I would like to flesh it out with some additional general Inquisition-scenarios during the siege.

So, if you have any suggestions what could be within the Inquisitions purview with a raging Chaos assault going on outside the Hive I would be happy to hear them. Cult-hunting is top of the list, but I hope you might have some other ideas as well.

Some possibly useful facts :

The Hive is situated by the sea and otherwise surrounded by ash deserts and mountains. It has two harbours and a air/spaceport.

It's a mixed Chaos force, so any of the Powers may be involved.

The planet is Roman-Egyptian themed, with a decadent pseudo-Roman aristocracy and a suppressed pseudo-Egyptian worker/trader-class.

The situation looks grim, with no news about possible reinforcements, and only a tenuous contact with other planets in the system. Starvation, disease and a brutal painful death at the hands of insane Chaos minions looms.

Thanks in advance!

You might have squabbling aristocrats - family members wanting to use the (non-warp) chaos of the invasion to topple their leadership, either because they are hungry for power or because they perceive their current rulership as inefficient, incapable of holding off the invasion or even already tainted by Chaos.

Servant class uprising - if the city is under siege and likely to fall, what are they still serving for!? Time to raise up and live like men before the end comes! Is someone manipulating this for outside reasons or is it an honest revolution?

Grand executions - what better way to keep the worried nobles in line than to publically execute chaose servants? Mind you, when you don't have any real servants of chaos on hand it works just as well to manufacture some ... and perhaps get rid of a few rivals in the process.

Riots - chaos without breeds chaos within ... people are literally losing thier minds to fear and panic.

Mass Hysteria - when put under stress the human mind finds all sorts of ways to cope, some of these people are seeing visions of saints, bleeding statuary, etc. throughout the city. Are these real visions or merely hallucinations?

Turtle Defense - on the whole the majority of the cities skilled fighters serve the ruling elite ... but some of those elite have become terrified and are holding up in their homes, turning their forces on anyone who comes near.

Mutation - the mass chaos force is sending ripples of corruption before it, causing mutations of living beings and structures throughout the city; how can you protect people from such a thing, and what will be the social fallout?

Religious Fervor - people who have mostly been paying lip service to the Emperor for years suddenly have a reawakening of faith. Led by charismatic priests these new born zealots are a force to be reconned with ... and some of those priests have wanted to make social/political changes for a long time.

Disease - disease is running rampant in the city, perhaps an effect of the chaos legions outside the walls, perhaps something else. The local law enforcement already has its hands full and now must try and contain the outbreak ... but everything they think they have, it pops up again somewhere else. What precisely is going on?

Public Justice - like the nobles, the common people need someone to strike out at and begin forming mobs to turn on anyone they deem questionable. Public burnings, hangings, dismemberment, and disposal over the walls is becoming common place.