I guess the sigil you refering to is the 1 hold by Baal. First get back to the mine building in broken valley where you can find william the guild master. He will give you a key to unlock the secret passage where you can enter the mine. After defeating Baal in the mine, you will get the sigil. I ko it at lvl28 while Baal is lvl30 and lvl up to 29 once defeating it. You will roughly get bout 13k to 15k exp by killing Baal and getting the sigil.
