After the spiders are taken care of, the party can move down the tunnel for another hundred feet or so before it branches in a north branch and a southeast branch. Today we'll talk about the north branch. This winds north for a good fifty feet and opens up into a cavern. It looks as if, at one time, there was a small pond in the middle of the cavern, but has long since dried up. There are stalactites and stalagmites through out the room, making it hard to see across the cavern from edge to edge. The roof is about twenty feet in the air, and with regular light sources a person can not see all across the cavern.
A DC of 25 will allow a person to notice light at the far side of the cavern. (+5 if they are not standing in their own light circle.) On the far side of the cavern is an outpost of Drow, (or the equivalent in your campaign), who need to make perception checks to make sure they are not surprised by the party. In my world I don't have Drow, so these would be rogue elves who do not adhere to rules of society, but I know a lot of people prefer Drow as bad guys. Make the Drow party a challenge for your party. I would give them levels and make sure their numbers are a good challenge.
There is the standard treasure, there is a map of the tunnels with cryptic marks on the map, (they mark where the "Drow" camps are), and a note that states they are to wait for reinforcements to attack the nearby House and capture one of the royal family. There is also camp supplies that can be plundered.