On Tuesday I ran Songs of Erui after about a month of down time. The party had traveled to the mostly-dwarf city, Cindervault, looking for the second fragment of a song that's been the primary focus of the campaign since the ass-end of the first adventure. When they arrived they found that it was mostly abandoned, but figured out later that night that it was now under the occupation of a shit-ton of drow. Not wanting to end up as spider-chow, they made a beeline for Cindervault's fortress under the assumption that something really expensive would be locked up there.
En route they ran into various drow entries cribbed from the Monster Manual (de-leveled of course to account for the fact that they were only level 9), in addition to a web golem, blade spiders, and some summoned mezzodemons (a drow demonologist needs him some demons). Now, after a few encounters they got to choose which way to take trying to get into the castle: the main gate or the access ramp for wagons and cripples, and deciding to tackle the main gate. Unfortunately, the gate was guarded by a purple dragon that I had to change to a black dragon after I realized that purple dragons go as'plode in the sun (WHOOPS), but in the end a summoned unicorn took it down and they got a level for their troubles. The session ended with the party on top of the ramparts observing 20+ drow and a drider communicating to a formorian king through a visual- and audio-only window.
That was all just pointless recap. The actual point to this post is that the following session they decided to stick to the wall and attempt to loop around the side of the walls where they hoped drow wouldn't be skulking about as they so often do. Now, my plans were to have them more or less make a direct assault, using the various siege engines on the walls to give them an edge. I did not think that they would just side-step all the encounters I'd had planned. So...time to wing it.
There was a lot of wall to cover, and several towers, so I used towers as indicators for where encounters could still rationally occur. It was just a matter of rapidly populating them in the span of a few seconds. Blade spiders were level 10, so in the first tower I just had one using it as a lair. Inside they found that there was one lurking on the ceiling amidst the suspended, putrefied corpses of dwarves. One of the characters opened the doors, and they had the druid go in under the guise of a Medium-sized spider. Discovering the spider, she tried to communicate with it, and trick it into thinking that there was a threat just outside. A few Bluff and Nature checks later, the druid was able to distract it long enough for the ranger and cleric to take a few pot-shots at it.
The highlight of the combat was when the druid used pounce to latch onto its face and pull it off of the ceiling, and then the ranger made a very nice grab and bull rush checks to slam in onto a ballista, which the cleric used to launch it off of the ramparts, taking a total of, "enough damage to say it died without bothering to roll."
That was the first instance of the session where I had to make up rules on the fly. How much damage does a ballista do? I dunno, 3d8 plus Intelligence modifier? How do I resolve the attack? Make a level plus Intelligence modifier attack. I didnt want to make it just deal a shitload of damage, for fear the party would try and lug them around and potentially backstab shit with them. So, going off of damage that ogres can dole out with boulders, I think it was a nice benchmark that made it worthwhile. Anyway, it wasn't the bolt that killed the spider, but the push effect that knocked it over the edge. A fairly easy kill, but all those nice rolls (including the use of memory of a thousand lifetimes)...fuck it. It was awesome.
The second moment came when the party decided to look for hidden passages into Cindervault so that they could avoid taking the door. I thought, sure, fuck it, give me a roll. After a Perception of 33 I was like, okay, you find a narrow crevasse that a halfling or gnome could safely get through. I figured that they could squeeze into it and that I could throw in some insect or vermin swarms to mix things up. Nope, not gonna happen: the druid used some level 10 daily that let her turn everyone into ferrets. Or rather, spider-ferrets (its D&D, animal combinations happen) and rapidly scurry through the opening.
And that was last night's session. The players easily steamrolled my encounters through liberal use of bull rush, push-effects, and clever thinking. Easy XP, awesome session.