Well they know that this will be the last time for the mercane for a while. I'm treating the mercane box like a bracelet of friends. If you're not on the same plane, you can't access him. If anything, you'll get an answering machine.
They're working their way up to defeat a god of the apocalypse. I saw a template for the apocalypse swarm from the 3rd party book, Advanced Bestiary, and am adding it to a raven swarm. It's going to be a very annoying CR 7 for them. I know it sounds low, but they're currently without much healing because of the last battle. The splitting ability that basically mimics that of a black pudding, and the fact that they only take half from slashing and piercing (which most of them have), is probably going to nickel and dime them more. Later that night they'll be taking on two goblin mobs. I figure that it's probably the best way to let them realize that it doesn't matter that they have an AC of 28 when you can get hit so easily anyways. The vampires should do the same thing with their rat swarms in the later adventure.
I should probably explain the last battle. They were fighting evolved soldier skeletons that had cones of cold, and blew up when you beat them. The reason why it was "annoying" was because the PCs kept getting nickeled and dimed by their foes. Slowing lowering them to almost 0 hp, but not quite. Then they'd fast heal, and nickel and dime them more. That was until they had them blow up for good.
Part of my problem is that not all of my players show up all of the time. Right now the cleric shows up once every fourth session, if he's not dealing with exams. Another guy's got two jobs, and a third's got two part time jobs (one's practicum) and full time school. We're only ever guaranteed to have three PCs, and an NPC at any given time. Most of the stuff I have planned is based on the fact that I'll only have four people that day. However, sometimes there's days when everyone shows up and we have all seven. So how this becomes a problem is when I see everyone, I immediately try to adjust for them, and add in more bad guys, but somehow it doesn't always seem to be enough. There was one time where it was way too much, and I nearly did a TPK, and ended up getting local NPCs to join in as if it was a bar room brawl. A little bit of divine intervention on that one. And it's not like I don't know the rules for what I should be adding. It goes into that whole realm of whether or not the CR is accurate for that particular party. One CR 6 might be as easy as a CR 3 if they've got the right kind of gear and spells, but another CR 6 might as well be a CR 10 to that same party.