Collect all 43 different Epic Runes and have the Dispatch ability. You must have at least 20 Captains alive; which is done easiest in Endless Challenge and Test of Wisdom. Select a Warchief that you will brand later. Send death threats to all the Captains and Warchiefs. Brand the Warchief that you selected earlier. Brand the 20 or more living Captains. Command all the Captains to become bodyguards of the Warchief that you branded. Summon the branded Warchief. Delete the Epic Rune to be duplicated, or it convert to Mirian. You should now have 42 Epic Runes. Kill all the followers. Upon death, every Captain and the Warchief will drop the same Epic Rune that you are currently missing.

Note: This will take some time to prepare.