The Sigil is held by a demon buried in the quarry/mine (the same one that was closed off by explosives).
You CANNOT get it until after you have the Battle Tower and dragon form, so it is impossible to get before Broken Valley is destroyed. You need to fly to the southern-most southwestern corner of the valley. In one of those Black Ring walled-in encampments, you'll find a locked door on the side of the mountain. Go in through there and it should be straightforward enough. Make sure to mindread the demon before the battle, as he gives 2 skill points.
Even though you can do the guild leader rescue before the Battle Tower, the mine is simply inaccessable, and even if by some miracle you get there (like via the floating glitch), I can't imagine you could beat him, seeing as its Lv30.
