So it went well. I was running with a group of 3.

Tarl- adeptus arbites seeker

Reddin-Imperial guard assassin

Zarkov -Adeptus Mechanicus Chirurgeon

Weird group. I know and small so I tronned it down abit or so I thought... So here is what happened. They meet up to do some roles to get some new things etc all the boring stuff before the adventure. They decided to take the identity of investigators from off world.So they went to the crime and did the normal stuff but for some reason they felt like the local law wasnt doing enough. After questioning the sanctionary there they knew of other xenos artifacts that were found throughout the hive and they called the sanctionary idea of staying behind the purity gate more than trying to persue the underhivers they thought they were being lazy. They decided after pissing off the women who is in charge there they decided to go to do actual investigating. Thats where the trouble began. They found out the first guy was a rouge psyker and tried to bring him in but felt like the sanctionaries there were incompitent so they wanted to bring them to the local arbites. Ugh. I try and tell them that they can handle it but they said that if they cant stop a mass murder how could they stop a rouge psyker. So they brought them all the way up the hive to the local adeptus arbites, Which I tried to tell them would take to much time and that the other houses might find it strange one of their own was taken in. They said that the rouge psyker has to be sent to the black ships and so they felt the need this was more important than a mass murder. So they traveled up the hive which is supposed to take days and according to the story that the zombies come out in like days also so.... They waited for the black ships to come which was almost instant bc they are prob more mysterious than the inquisition and they decide to go back down. Turns out they spent to much time and the zombies were at almost at the gates and the local sanctionaries told them that it was his older brother and he unleashed a horde of zombies and that they had orders to close the gates and told them if they could hold off the zombies until they can do so. The players have decided since it was their fault they felt obligated. Now they are on the other side of the gate with zombies everywhere. So.... What now? lol. The sanctionaries told them to hold off the zombies and or to try and move them to a different place and ya thats where we ended it.

My question are thus. Should I basically run a zombie Apocalypse scenario where they try and survive in the under hive with zombies and mutants and gangers while trying to still stop the older brother as part of their original mission?

Well, first things first:

if your characters want a Rogue Psyker to be taken by the Arbites and that derails the plot, have the Arbites COME and get that Rogue Psyker. The characters would have only needed to "guard" him for a day where they are and everything would have been fine:

Second, it don´t think that it takes days (plural) to travell in a hive when you are rich enough to effort the means for it. Just think about where a modern train can take you in a day. One day, okay. DAYS when you have stuff like tube trains and landspeeder-like vehicles at your hand (which the Arbites surely has). Nope!

Back2topic:
So, you people basically skipped the investigation, you have already established that "Gantry" is overrun. And the officials know that this is a "Zombie Apocalypse". Now, you have reason to send in a couple of Arbitrators (or simliar forces) along to push back the undead. Push them back to "Gantry" and have a fight there. It won´t be to different from the fight mentioned in the gantry since this "wave" of undead is very likely to be just that: the first wave that was send for testing. Have the ritual to be still going on. Have them "clean" Gantry and find some survivors. They can question and interrogate them, but due to just having escaped a fate as zombie-chew, they might spill the beans rather willingly (terror has such effect on people). They might have hold up in the gangers base off operation, a higher level of Endish Manor (or have that one being collapsed down into the sump) or in any other stury building.

From there on, "business as usual". But you will have a score of Arbitrators around and should feel free to beef up the zombie presence accordingly.

Well, first things first:

if your characters want a Rogue Psyker to be taken by the Arbites and that derails the plot, have the Arbites COME and get that Rogue Psyker. The characters would have only needed to "guard" him for a day where they are and everything would have been fine:

Second, it don´t think that it takes days (plural) to travell in a hive when you are rich enough to effort the means for it. Just think about where a modern train can take you in a day. One day, okay. DAYS when you have stuff like tube trains and landspeeder-like vehicles at your hand (which the Arbites surely has). Nope!

Back2topic:

So, you people basically skipped the investigation, you have already established that "Gantry" is overrun. And the officials know that this is a "Zombie Apocalypse". Now, you have reason to send in a couple of Arbitrators (or simliar forces) along to push back the undead. Push them back to "Gantry" and have a fight there. It won´t be to different from the fight mentioned in the gantry since this "wave" of undead is very likely to be just that: the first wave that was send for testing. Have the ritual to be still going on. Have them "clean" Gantry and find some survivors. They can question and interrogate them, but due to just having escaped a fate as zombie-chew, they might spill the beans rather willingly (terror has such effect on people). They might have hold up in the gangers base off operation, a higher level of Endish Manor (or have that one being collapsed down into the sump) or in any other stury building.

From there on, "business as usual". But you will have a score of Arbitrators around and should feel free to beef up the zombie presence accordingly.

They did do the investigation. They just did a really bad call. I even told them in the after game talk that they made the wrong choice but I decided to still role with it. When they talked to the rouge pskyer they felt that he was behind the attack and not the older brother. I told him that if you strongly believe that let the local arbites handle it since they are used to it. They said if they let a rouge pskyer live for so long they felt that they wernt doing there job even though as an arbitrator I told them that the other arbitrators were busy discovering and dealing with other xenos artifacts recently and so the players felt that they were getting stretched way to thin than and thats why they wanted to do it personally.

But on another note I like your idea. They could still do the investigation in a way but ill just make the pcs who are still down there among the survivors or even more murder but not by undead but by a bullet or a bolt round like in a battle as if the older brother took out his adversaries to prepare for the ritual. I can do as you said and still have them do the battle and other things so it might not be so bad than, Thanks for the advice. I was going to do something like that and ya now I could "flesh" it out. lol.

Also, feel free to let the players suffer the consequences of their actions. It may lead to other adventures. Another consequence may be that the local authorities refuse or make the player's lives miserable due to their allegations of the local authorities being lazy and not doing their jobs.

Any consequences good or bad teaches the players that they do have a potentially large impact on many levels.

Well, Pha, how did it went?