You can only attack The Overlord with Energy.
|Name (Tier)||Cost||Notable Drops||Total HP|
|The Overlord||Free||Black Crystal|
You can sell the Black Crystal he drops to The Balin Miners for 50 Platinum. They are located west of the Windown Mountains. You can also keep the Crystal and use it to Enhance items in the Kirsinti Treasure Shop located in Book 6.