Demontamer’s Sash can be obtained on the North of Royal Meadows of The Holy Empire of Sanbreque, Northreach. Head north to the Baywatch and you will see a tower ruins with a Treasure Chest. It is better to defeat the enemies first before getting the treasure.
While slaying a demon merely returns its soul to the hell whence it came, until such time as it is resummoned to inhabit a new host, taming a demon is a much more effective means of dampening its evil influence on the corporeal plane. By luring and trapping the undying spirit of hellspawn into accoutrements worn on one's person, one might also harness their power and channel a portion of it as one's own.