hamishspence said:
My best guess was start with fire titans lava: 4D6+6 + Immobilized (save ends) maybe every turn.
Oh, you can certainly improvise it. However, I was commenting more on the ease of finding this information. Lava rules might be under "terrain", or might be under "environmental hazards" or, since this is an exception-based game, they might be hidden in some power somewhere. To know that it isn't covered, I had to read the whole of the PHB and the DMG (which, as has repeatedly been pointed out, no-one actually does

).
(That the DMG doesn't even give ball-park advice on how to cover this is rather poor. It strikes me as being very good on general advice, but shockingly poor on specifics in many cases. But that's another thread.)
Oh, and yes, this has cropped up in actual play - in my Shackled City campaign a PC fell into lava. The contrast with the 3.5e DMG is stark. There, 'lava effects' is listed in the index. Moreover, such is the quality of that index, I can be reasonably sure that falling onto glass, for example, isn't covered by the rules - the same is not true of the 4e DMG.
Or how about fighting monsters with gaze attacks? The rule that blind creatures are immune to gaze attacks in clearly marked in the MM glossary, but what about closing your eyes? Such a character isn't really blind, but does it count? Well yes... but that's only covered in the 'Medusa' entry!
Then we look up 'blind', which isn't in the PHB index or table of contents. However, 'conditions' is listed in the index, so that's fine. Until we find that there's a sidebar on a separate page that deals with targetting creatures you cannot see - and it's only in the ToC, not the index!
I'm sorry, but this is not conducive to finding things quickly when they're needed.