Bagging the Bagman

Every adventurer has heard some version of the tale while around a campfire. How a bag of holding left uncared for summons a horrible boogey man, a creature weilding forgotten gear, and who craves the lives of those who desecrate its sacred corner of the pocket dimension. Those were only ever stories though right? Well I hope for your sake they were because adventurers in this mountain side town have been going missing and the townsfolk have hired your group to get to the bottom of it all. The only evidence, a bag of holding left open on the previous adventuring parties night stand. Do you dare investigate further?