Has anyone else used a titan as a plot point in one of their games. I'm trying to decide if I should stat it as a vehicle or just use it as terrain. Context: the characters are trying to break into it while its in motion in order to kill the princepts/bridge crew before it can bombard a small hive containing an important Imperial holy site.

I wouldn't stat it like vehicle. It's just plain too big and powerful to be considered like that. Decide what it requires for them to get on board and treat the interior as a location that they move through complete with badguys etc.

So for example:

Awareness to spot a suitable boarding point.

Climb/acrobatics to actually grab on.

Climb to get to an entrance

Security to break in

The Black Sepulchre uses a titan in it, not sure how detailed it is, but if you can get a look at that then it might help. They'd certainly get major penalties for moving on the titan while it's in motion.

it depends on the titans class, warhounds could be stated as vehicles. anything bigger I would use the advice above. if I'm not mistaken there are stats for a warhound in deathwatch's "rites of battle".

I agree with BrotherKane . Stating a Titan for use against DH Acolytes would be pointless- they are intended to take on whole tank columns! I would treat it as a 'dungeon on legs', and have the mission determined entirely by what happens inside the Titan.

Even a Warhound titan is very , very, VERY powerful. My players annihilate Daemon Prince in just a few rounds. ..

Thanks for the Advice guys!