Hello there,
I need an idea and am sure you can help me find that idea: I want my players' characters to be unwittingly/unwillingly lured into the Underhive and to get lost there. They'll already be in the lower Hive's lower levels so it is shorter for them to suddenly find themselves in a dark, unknown and uninhabited area they don't even know.
But I can't find any better clue to drive them there than the usual assassin who tries and misses killing them (too cheesy) or the common pilferer who bumps into one of them to steal his money (which raises another dilemma: where do imperial citizens keep their thrones [mainly bills]? I don't think they sport their pouches like in the usual fantasy settings).
Anyhow, in both cases I could only hope they rise to the bait and start following the NPC, whose trail they have to lose.

I'm not satisfied by these hooks: can you suggest me better clues?

As for the situation, they came in Nova Castilla (Fenksworld's capital) a short time ago and have just found out what happened to the previous acolytes, who had discovered the Beast House's net linking Fenksworld's three hives: my players' characters have yet not even a clue there's a Beast House in the planet, they only know their forerunners are dead. They'll learn of the Beast House once they'll be forced to go to Volg for a different task.
Oh, and when they'll get rid of the Beast House I plan to have them investigate a Logician cult: it's quite a long run though.
All but one (the arbitrator) come from other worlds; the arbitrator works in Nova Castilla's middle Hive and doesn't know deeply the lower levels.

Thank you for any help!

In my own campaign, I've sent the party into the Nova Castillian underhive a few times: once a PC (a Scummer, naturally) was hired as a driver on a shipment of stolen goods being handed over to smugglers, the meet aranged on the edge of the underhive; the second time, they had to rescue a contact (who had vital information) from an underhive gang; finally, they tailed a suspect to a meeting of conspirators that was being held in a long-abandoned opera house in the depths of the underhive, to avoid detection by the authorities. Would any of those possibilities work? If not, there is always the old cliche of the Treasure Map, possibly leading to a stash of Archeotech (or more likely, the map is a fake intended to draw would-be looters into a trap...).

Adeptus-B said:

Id'd say no, for at the moment both their "to do" and "bad guys'" lists are almost clear.

But with the "meeting at the long-abandoned opera house" you've given me a good idea: I'll have them notice a strange looking person who's moving in a suspicious way. I'll stress his looks' and attitude's description to invite them follow him: and when they'll lose the trail it will be very difficult for them to find the way back.

And yes, I need them to get lost so they can stumble in some archeotech.

Thank you for your help!

Another option: Make them shadow/follow someone to begin with

Someone previous groups of acolythes where seen with/linked to. Make it there mission to follow him around to find out who are the contacts of said guy (if any).

Then, send the guy into the underhive... and make him notice the fact that he is followed so he gets off his regular route into dangerous territory just to get rid of those who dare to shadow him

If the pc are not big into shadowing, give them a "bug".. and make the guy find the bug and let him attach it to an oversized rat the will go where you want the pc to and up..or something...

One more idea on the pile:

If they're already in the lower sections of the hive (close to the underhive/lower hive border) then maybe have one of the PCs fall though a hidden or poorly-patched hole to the level below. You can have him break a leg or something if you like, or you can brush off the idea of injury with a good old Hollywood inspired "fell into a trash bin" or something like that. (Not saying it's realistic, just saying your PCs probably won't question it as an excuse for not dying from the fall.)

The other PCs will naturally want to save their friend, but that will likely involve going down to get him (especially if his leg is broken.) Whatever means they use to get down, make sure they can't use it to get back up again, so now they have to go a-wandering to find another way out of the underhive. I'm just thinking, if your PCs are full up on things to do and people to kill, a little "random" bad luck can be a good way to inject more obstacles.

If one has become clear in my many years as GM/DM/ST (etc.) is that players are rather fond of one thing, one thing they would kill, pillage, anything for and that is money.

Perhaps some underhive scum wants to bribe them to look the other way, hire them, but make it smell fishy...they'll find their way...