Hall of Infinite Doors

"What does this key open?" you ask the dwarf. "Er...I don't know. I just found it." the dwarf's little feet dangle in the air. You shake him until he tells the truth. "Okay! Okay!" the dwarf shouts, "It's a key to a door. A door in the wall of that chasm down there. But it is very difficult to get to." You examine the small sword symbol engraved on the key. What door could he be talking about? And where would it lead?