However, this component is the most difficult to drop, as are epic class monsters. You need to battle at least 58 stages so you can receive Void Core in random rewards. It can also fall into the events of Tower Assault and the fight against the Dragon, however you will need to be on the level (data needs) so that the Void Core begins to fall, being able to receive up to two when you are on nvl 55 in these events.
The Void Core can also be bought in shops in the crafts section, along with a set of equipment that can cost between 1350 Gems containing 2 Void Core and other components and 2750 Diamonds to have 10 Void Core and other components. Sometimes it has a Promotion of Void Core, costing 150 per component, however it is limited and can only be bought once, although sometimes I came up to 5 Void Core. You can also use real money or Ninja Kiwi Coin to buy several Void Core.