Destruction King is one of the evolution options after an Apostle Lord (that Rou combines with another evolution option in the light novel) reaches max level and fulfills the right conditions.
The conditions to evolve are to collect 5 ‘Secret Keys of Destructive Devil King’ available only in The Tower game mode (similar to Challenger's Cave) where players can organize up to 9 units in a formation to challenge waves of enemies. Secret Keys are available to be purchased in the tower store with points earned. Secret Keys can also be obtained via chest drops after reaching Iron rank, starting with a low drop rate rising after earning higher ranks. By accumulating certain points you can raise your rank (Spending points does not lower your rank).