Quote Originally Posted by Metastachydium View Post
As for the spread, well, there are spells with areas measured in miles (Eclipse comes to mind, with its 5 miles radius), so this is more of a balance problem (those spells are, of course, epic spells) than a problem of stuff being poorly quantified.
I was thinking you should quantify the spread of the gas so there's an explicit event created when the monster dies, where the party has the chance to evacuate as much as they can from that 1 mile radius as the cloud expands behind them. Where a nuke is basically instantaneous and then the remaining radiation deals damage per round, if the cloud spreads at a rate of 100 feet per round (or whatever number), the party explicitly has time to try to save... whoever they might have the chance to save (including themselves) before it reaches them.

Although I admit this might not fit what the creature in question is actually based on.