Apparently a fevered imagination is more creative, I've had a fever for the past few days and I just came up with an idea for a weird magic item. Course it probably has something to do with watching Hikaru no Go on Toonami Jetstream, but whatever.
Ok, so this outsider of some sort, probably evil, stole the souls of mortals and was a general menace, then one day he was challenged to a game of Go by a mortal. Cocky as he was he accepted immediately, but he lost and was trapped inside the Go board. Then the mortal went and hid it somewhere and that was the end of it for a long time. The fiend was pretty pissed because all he could do while trapped in the board was play Go against all the souls that he had trapped and were now trapped with him. Of course he did get pretty good. Also, he eventually figured out how to tap into his victims' intellects' and use them to increase his own mental abilities. Sweet huh? So if anyone finds the board the fiend will try to speak with them and convince them to release him, so on and so forth. Also, if someone plays Go on the board and loses their soul get trapped.