First I'll relocate the meeting hall (with the gear in a room behind it) far deeper into the fortress. The entrance to this hall will be locked by two (artifact) floodgates.
The trial is the Trial of Agility. You enter a long hall with a door somewhere along the center, which requires you to step on a pressure plate in the beginning. This opens a floodgate on the other side of the corridor which releases magma. If your agility isn't high enough, the magma reaches the door before you do and you are consumed by the heat. The next room will have a pressure plate which opens the first floodgate. There's also a lever which lowers a drawbridge that allows you to enter the room before the floodgates again. I'm thinking about leaving it permanently inaccessible if you fail, to increase difficulty, but there's also the option of a draining system.
With the opened floodgate, you may now enter the Trial of Toughness. You climb up a tall tower, and have to survive a fall of ~6 (may require thorough testing
) z-levels. Once you're down there, there is no way back up. After that, you walk down a 1 tile wide corridor which forces you to trigger several gore, blunt and slash traps. Having several broken bones from the fall (probably), you'll have a harder time dodging them. At the end of the corridor there's another room with a pressure plate which opens the second floodgate. You can leave this room in the same way as before.
You now have access to the Trial of Strength. In the corridor leading to the meeting hall, there's a 5 tile long (and as wide as the corridor) open space, filled with magma. Pulling the (artifact) lever will trigger a cavein that fills the gap, allowing permanent access to the tentacle demon infested meeting hall (bridges could be exploited
). Now, if only there was a way I could get all the demons, including their leader to hole up in there..
Defeat the demons, get the equipment, rid the world of the subhumans/dwarves!