I was DMing for a large, tough party (7 x 4th level), so I decided to amp things up a bit. The party needed the hoard, I had "visioned" the cleric that a scroll of
Hallow was part of the treasure, and was needed to cleanse the chapel in Cragmaw Castle.
I made Venomfang's tower defendable, the building was roughly repaired*, albeit the top of the tower was hollow, allowing Venomfang to fly in and out as he pleases. There is just enough room in the building for Venomfang to move around.
The windows are boarded up.
*I didn't explain this, but if necessary, I would have had it done by the Cultists, trying to curry favour with Venomfang.
The party wasted no time with the Cult (I was disappointed!). They just pointed at the obvious-looking bad guys and blew them away.
So they skip over to the tower...and Venomfang has spent the last few rounds breathing inside the building....the party can see green gas pouring out of the holes and cracks all over the building.
So now the party has a problem, I am not having the poison gas simply dissipate, rather it hangs around for several rounds. They will take a small amount of damage every round they are within 5' of the building, and lots more every round they are inside the building.
The outside of the building was swarming with ankle-biters. If you start your turn near the building, roll a 1d4. On a 1. nothing, 2-3 one twig blight, on a 4 two twig blights.
So the party couldn't just stand there, they had to force their way inside. They broke through the door (well the raging Bearbarian did, since he was taking only small damage from the poison)...only to find that Venomfang had another nasty surprise waiting for them.
Inside the doorway were three (IIRC) ash zombies, trapped in a jerry-built cage. The zombies are immune to the poison of course. This was the inspiration for the cage (actually just heavy timbers that Venomfang had placed there):
So now they have to get inside the entryway, kill the zombies, and then dismantle the barricade, all the time taking moderate amounts of poison gas damage.
And when Venomfang himself gets going, actual breath attacks as well....
The party did have some things going for them. I hadn't reduced the potency of Sharpshooter at this stage of our game, so the archer was having fun popping shots at the bulk of the beast through the narrow (arrowslit-style) windows in the tower. Eventually Venomfang had to do a runner, but he was actually killed mid-flight by the Sharpshooter at (normally-would-be) extreme range.
Epic fight, and I am really pleased that I jazzed up the fight in a meaningful way, while preserving the overall theme.