tylermalan said:
Well, yes and no, I guess... it depends on what you consider a "chance". They didn't roll any dice to determine how powerful it was, no. However, it is obviously huge size, and they know the reach. It basically looked like a huge, feral necromancer.
OK - here's how I would have done your intro differently. Take it with a pinch of salt, and YMMV and all that.
You are doing a horror campaign, and you want to foreshadow the beastie somewhat. You intend the players to do the CoC thing of Library Use etc before actually bearding the foul creature in its lair - and possibly to determine its strengths and weaknesses, and at very best to determine A Cunning Plan to make The Final Battle happen in terrain favourable to the PCs. You've got a number of Exposition encounters planned out - finding out what the sigil means, gaining more information from the scenes of later attacks, that sort of thing. So you don't want the PCs to go haring off and frontal assault the bad guy, whom they know from the screaming to have only a minute's head start on them. So you need to give them reasons not to follow - fear is a good one. But a more substantial obstacle is probably necessary.
So, first rule - Show, Don't Tell. You hand out the sheets for the NPCs who are actually guarding the assassins, and say that the players are going to just do a quickie one-off for this evening. Then you have the fight, where you are notably short on detail about the baddy, emphasizing the poor visibility in the flickering torchlight, particularly as one of the oil lamps was put out when the floor smashed upwards. The NPCs all die, leaving the players knowing only that it's big, it's bad, and it's scary in the dark. You do it with legitimate die rolls, although the pregenned guards might be a tad weaker than the real PCs, but regardless, the *players* should have to go through a TPK where even those that try to run are brutally cut down as they scrabble to open the locked door. That should drive home the badassitude of the murderous opponent, so that the actual PCs do not need to risk actually engaging it just to determine how powerful it is.
The various clues are left as required, if necessary by an extra NPC whom you play and who /just bleeds to death/ just before the real PCs enter, but not before quickly scribbling the salient points in his own blood on the floor. When the giant finally retreats down the hole, it certainly collapses the tunnel behind it. You don't want the PCs going after this murderous killer in the dark when it lives down there and they are non-optimised for the adventure. If you haven't tailored the adventure to their eventual characters, then you have to cut them a break, or they will just all die - it's the way of D&D, which is a heavily metagame-dependant tactical system that your game is using the tactical elements of. No optimaxed builds means you cannot expect the PCs performance to be anything but below par when compared to the EL vs CR metric - and as a GM, you go to war with the party you have, not the party you might want, if I can paraphrase. So, rocks fall, everybody lives!
The PCs *can* dig through the rockfall, but it will take time in which the giant will leave the immediate area - and perhaps the NPC guard captain will offer to get a work crew in to do the excavation, which will miraculously be finished just about the time the Library Use is complete, and the PCs find the next killing, and maybe have had a chance to slot some passing goblins or something and maybe level up or pick up a natty scroll or two of some 2nd level spells that might be useful...