So we're going through the Redbrand Hideout in 'The Lost Mines of Pahndelver' and the players were so terrified of the Nothic that they cut a deal with it to keep bringing it a supply of dead bodies.
This bothered the fresh-faced paladin, who sees herself as a story-book hero despite the realities of the world; and the druid npc, who hates messed up magic stuff on principle. So the two of them are working behind the backs of the rest of the party to find a way to off this thing, even going so far as to ask for help from the druid's old master.
My issue is this: The party could have taken the thing down even at their level. It would have been a tough fight, make no mistake, but it would have been doable. So when they finaly do face down this thing (with, possibly, the help of a more experienced druid) should I keep the creature's stats as are to let them know how easy it would have been? OR do I tweak it to keep it the nightmare fight they suspected that it was?
Excellent question, and honestly, there's no right answer. You're going to get entertaining results either way. My personal recommendation would be for the druid's ally to have some knowledge about the creature, and to say something along the lines of, "The nothic is indeeed a dangerous creature, but alas, my duties keep me here. However, I know what you are capable of, and this is a problem I think you and your allies can handle." If they already spoke to the druid and you portrayed him as not knowledgeable about it, he could have done some research with other allies to bring this information to the party. By way of additional aid, maybe he sends them along with a healing potion (if you're feeling generous).
I generally don't change monster stats in response to developments in the campaign, but as you said, they're expecting a nightmare fight and this is one of those opportunities where I might like to meet their expectations. The extent to which I would buff the nothic depends on three main factors.
1) Whether the druid ally accompanies them 2) How much of the hideout they cleared out, i.e. whether the nothic is the only fight for that adventuring day, indicating how much of their daily resources they can afford to bring to bear on it 3) Whether the party has gained any other notable resources (levels, equipment) since they last met the nothic
Again though, it could be just as entertaining to leave the nothic as is, and measure their surprise when they drop it handily. I don't think you can go wrong, whatever your final decision.
Or, when they go back have the nothic just not be there and have it tease them from the shadows for the rest of their lives. Nothics can read your thoughts right? He'd probably know that the druid and paladin had it in for him, and he would not be caught unawares. He can continue to be scary so long as they never fight him!