This is a spider encounter I built and tossed at my group a while back. Since this is home-brew, you can tinker away to make something fun and challenging.
Trap Door Spider
The spider has a sticky trap waiting at the bottom of its concealed pit.
(Use CR appropriate for your party, in my case I used the Ogre Spider stats; tweak the DCs as needed as well)
The Trap Setup:
15'x15'x15' camouflaged pit, with webbing throughout the bottom and sides, those who fall in (failing DC20 reflex save) are considered entangled and unable to move unless they make a DC17 STR check to rip the web free in that square. They could also use fire, but will take 1d6, and need to make a DC17 reflex save or the web -and- their own cloth materials catch fire doing another 1d6 per round until they spend a full-round putting it out, or dousing with water/using the create water cantrip, etc. On a successful STR check, they can move into the next webbed square, basically allows 5' of movement per move action if successful on the STR checks. Once a square has been ripped free, it is considered clear. However, the sides area also webbed, making climbing out nearly impossible. Of course fire could be used here, or toss a rope and set a Str DC for those on top. If you don't want fire to be the easy answer, do this encounter during the rain (or winter snow-fall as was the case in my game)
Monster Tactics:
The spider itself comes out of a crack (has Compression, and a ground based version of fly-by attack allowing it to use the rest of its movement after attacking). With the special attack, its able to pop-out, deliver a sting, and move back into the cave on its turn. The PCs can hold actions to take ranged or melee attacks when it appears. It'll first try to incapacitate anyone in the web, but once it reaches 50% or less HP it will come out one last time to attach a webline, and drag the PC into the crack while it has full cover. If the group can sever the webline, or knock the spider below 25% on that last exposure, it'll abandon the prey and stay hidden in the crack.
The PCs can still get XP for defeating it, and appropriate treasure in the bottom from previous victims. This is a good place to plant some kind of item (letter; strange statue; small chest with note from local noble and gold for services to regional giant clan)
This ended up being a lot of fun, only 1 PC (the main fighter) fell in, but eventually another PC had to jump in to help him, she ended up being drug into the cave, and crit-killed it with her dagger as she was on 1hp herself.
