Grimrock

Ok, I misread your post, thought you dropped it in front of the door, so it could have been the "item on floor" bug instead. But it was stupidity on my part since if you walked to the door with the sword, you would not had time to drop it before the door opened.
So, three conditions confirmed, we now know how this door is opened. However, the item on floor definitaly seems to be a bug, should be brought back to the dev.

Hey peeps... just adding my own experience to this. I've got pretty much everything in the game up to this point, but I couldn't get that bloody Nex sword. I'd been to do the door when I had 2/7 treasures. It wouldn't open. Found the 3rd treasure by accident (you know how), went to Fighter's Challenge door again, still wouldn't open (I knew it wasn't the treasure piece, I just got lost and ended up there again ). Then I got the Nex sword (that was ridiculous. I hate pits. About 20 reloads at least) went to the Fighter's Challenge door, and it opens. So regardless of what else I may be carrying that is required (eg Orb), the Sword of Nex is for sure required. Sorry if this has already been determined... thought I'd read everything about the door and it still seems a mystery for some. Not sure I want to go get my ass kicked considering what others have been saying. Wish me luck.

I now got to the door on my second play through. This time the door opened without any trouble when i had:
- The sword of Nex on me (Equipped)
- I had visited lvl 7
- There was nothing on the floor in front of the door

If I remember correctly, during my last game I had dropped a torch on the floor near the door, if the door doesn't open because of this it's obviously a bug and I'm going to pm a dev right after posting this message. Thanks everyone for your valuable input!

Okay just figured it out.
For spoiler reasons I only mentioned how to open the wooden door for Fighter's Challenge, nothing more.