As noted else where you need to talk to the cook (I think his name is Bryant or something like that) then talk to Rashaz - kill the rats - talk to Bryant again talk to Rashaz again (but can't remember if talking to Rashaz gives you any extra points or not - just finishes the plot nicely).
As far as the lava flow:
You need three items which you may, or may not, have picked up along the way but are not sure what to do with - if you still have problems try the spoiler. BTW this took me ages and ages to solve - only to find that the solution was on other pages/messages so you may like to browse these first.
SPOILER: <span class='standouttext'>Spoiler : </span><span class='spoiler'> go back to the first level you need to pick up a barrel of coal (its underneath the portrait) where you killed the torturer.
go back to the gay doctor - begind him are some potions (which you probably picked up) but also a dish of sulfur which you need now
return to the harrassed cook (Bryant?) behind him - that is slightly to the north west - is a barrel of salt
go back to the lava flow and play with these three items (try mixing) - when you have it right the solution is obvious </span>
Hope that this helps
marts