The party had explored the entire upper level of the Slaver's Stockade. Things had gotten quite weird the further along they went, with more and more sinister clues that the giants were involved. Nefarious purposes seemed to be coming from multiple directions, but what seemed lacking was any evidence of where the slavers escaped to. Carefully backtracking their path they re-inspected one suspiciously angled hallway and lo and behold a secret door!
And behind that secret door a heavy, black, velvet curtain obscured what was beyond. The Ranger sent his magical companion into the room to scout out the environs, and after a tense few minutes the small creature scampered back telling the tale of a strange octagonal room...
The party had entered from the lower right hand corner, the door opening inward into the hallway. The room is made of old, heavy stonework. An identical curtain covered the lower left hand corner (a clue that there is a second secret door leading downward into the dungeon depths below). On the floor in the center of the room is a octagram design etched into the floor and filled with some sort of immovable (and enchanted) metal. But what looms above everything in the 20' high room is a massive metal statue holding an impossible large steel sword. The thing may be an empty suit of impossibly large (9' tall) armor. As the party enters a dim purple light illuminates the area, as if coming from the very air itself.
Two other things occur as the party enters the room. They hear the secret door behind them close and lock. They also may perceive (DC 16) a hiss coming from the two upper portions of the wall on the left and right. Nothing else will occur yet with this, but a DC 20 perception check will reveal two small holes, one on each wall, at about 7' in height, angled slightly downward and the slight smell of "cave gas" (natural gas or methane).
This room is designed to be either a death trap or a challenging obstacle the clever party can perhaps foil with minimal damage. If the party avoids stepping onto or into the sigil in the center of the room it will have no effect. But if they purposefully step into it they will find they are trapped within the confines of the symbol they are trapped there. They may move about and attack with ranged attacks outside of the circle, but may not leave. They also will find that a powerful dispel magic effect exists within the confines of the octagram and that only nonmagical attacks will work. The possible exception is that if the person inside the circle has magical missile weapons they will be thrown as a nonmagical device, but will regain their magic once outside the device. In our case the Ranger, who is also a Psion, had the advantage of using psychic attacks with damage other than psychic (acid).
The other bad thing about being stuck in the center of the device is that it makes you very subject to the two flame traps that are positioned in the upper left and right walls. Every 12 seconds (every two combat rounds) a cone of flame, describe by the red triangles on the map will blast outward doing 3d6 damage for each cone (6d6 inside the octagram). DC 18 Dex save inside the octagram for half damage, DC 12 Dex save for the area in the cone outside the octagram.
The golem will attack at the most advantageous time, but will not willingly enter the dispel magic area. If he does he will immediately be rendered inert (since he is a magical construct) and collapse into immobile helplessness. He will also be unable to move out of the are of effect. Behind the Golem faintly inscribed on the wall are several magical glyphs. A DC 15 Arcana check or a detect magic spell will reveal the command word to inactivate the glyph in the center of the room allowing anyone trapped therein to leave, and removing the dispel magic effect.
Note the command word does not cancel the flame traps, as they are mechanical and triggered by the locking of the first secret door. A DC 20 perception check will reveal the very subtle button device set between two stone blocks by the inside of the lower right secret door. This button unlocks both doors (lower left as well) and shuts off the flame traps. The flame traps have enough for 6 charges (two combat rounds) before being spent.
Overall the room played out fairly well. The party druid actually cast thorn whip and pulled the Golem into the sigil rendering it inert--but they hadn't figured out that it was a dispel magic zone. They only knew the Ranger couldn't get out after he ran into it. He was still able to use his psionic abilities since they aren't magical. The idea, of course, is that the party figure out the are is a dispel magic zone and use it to get the Golem inside. An added wrinkle could be that flames can recharge the Golem--metal golems are recharged by fire damage--but a DM could rule that the recharge is a magical effect and not able to work in the circle. This didn't matter for my party since they had already found out flame recharge it when the mage cast a firebolt at it. So once it was pulled into the circle they pulled it to the ground below the bulk of the flames.
The challenge level for such a room is high and will vary depending on your group. But the Iron Golem alone is a CR 16 and the traps are between 7 (flame traps) and 10. The Dispel Magic containment trap is also high, since if you get a spellcaster caught in there, or a PC with a powerful magic item the party has just had their power majorly nerfed, so you could scale it between 8 and 12 depending. For a party of 7 at roughly 8th level I would say this was a hard encounter to just below deadly, depending on how they play it. It literally could be a death trap if they aren't smart and coordinated with their efforts.
As far as where to put the room, I like it as a hidden area that must be overcome in order to get to the next level or major milestone. Not necessarily the big boss, but it would work great as a Red Herring if you are using the 5 room dungeon design process (which is called the trick or setback in the link). Although it can just as easily be run as a guardian if you would like with some tweaks. For us it worked great as the hidden "A-ha! There it is!!" room which enabled passage to the second level of the dungeon, and the escape route for the slaves.