Trap Door Suprise

"Is that a door?" Nimble asked as they came upon a strange circle in the wall.  "According to the map it looks like the passageway should be right here."

"What's that design?" Rathgar asked Feris.


"Nothing I've ever seen before."  Feris mumbled a few words of power. "It isn't magic either."


Nimble tapped the circle with the butt of the pole, ran it along the grooves of the pattern, and finally around the edge.  Whatever it was, it was solid.  He leaned in closer "I don't see anything that looks like a trap."


"Should we just try to push it in, or pry it open?" Allianora asked.


Nimble shrugged, Feris shook his head, and Rathgar said "I'll do it."  He pressed his shoulder against the circle and pushed.  It shifted in slightly, and then pushed back hard, knocking Rathgar to the ground. Shifting slightly back and forth the eight legged creature emerged from the tunnel, standing over Rathgar.  

 Allianora and Feris back-peddled away from the weird spider, while Nimble scrambled to keep it's rear towards him, and Rathgar quickly worked his dagger free of it's sheath to stab up into it's weak joints.    

Shield Trapdoor Spider
Rules Cyclopedia

Armor Class: 4
Hit Dice: 5* (L)
Move: 120' (40')
Attacks: 1 Bite
Damage: 3d4 + Poison
No. Appearing: 1 (1)
Save As: F3
Morale: 6
Treasure Type: U
Intelligence: 0
Alignment: Neutral
XP Value: 300

Monster Type: Lowlife (rare)

Shield Trapdoor Spider
Mutant Future

No. Enc.: 1(1)
Alignment: Neutral
Movement: 120’ (40’)
Armor Class: 4
Hit Dice: 5
Attacks: 1 (bite)
Damage: 3d4, poison
Save: L3
Morale: 6
Hoard Class: VI

Shield Trapdoor Spiders are 8 foot long, black and brown monstrous arachnids with distinctive abdomens;   they are oddly truncated with a blue-green hardened disk that is covered with grooves and ridges forming distinctive and individual patters.  Those bit by a Shield Trapdoor Spider and fail their save will suffer the effects of the Confusion spell for 2d10 rounds before death. 
These spiders are aggressive and cowardly.  And all reaction rolls suffer a -3 penalty to the roll.

via:
Cyclocosmia - Wikipedia
&
 Let me take a closer look.... oh no wtf?!? - Geekfill