Hallowed Bars are a material dropped by the Mechanical Bosses. Defeating the Destroyer, the Twins, or Skeletron Prime will reward you with 15-30 bars. Once you have the Medallion, wait until nighttime and use it to begin the event.
Once defeated, he drops Souls of Might and some Hallowed Bars. 2 Gather equipment. Get Titanium Armor/Adamantite Armor with helmet depending on your class choice or Frost Armor/Forbidden Armor if you are able to farm Frost core/Forbidden Fragment. You will need at least 12 greater healing ...