Well first they would need to add fire breathing, which currently no dragon has.
Secondly they would need to make it have multiple attack types, like magic then sometimes melee.
then add attacks that poison and reduce the player's stats
after that perfect the dragon by adding this:
The King Black Dragon has 3 unblockable breath attacks: Poison, Shock, and Freeze. His normal dragonfire has a max hit of 65 with no protection, 15 with Anti-dragon or Dragonfire Shield, and completely blocked with both the shield and Antifire potion. The KBD's dragonfire is significantly stronger then normal dragonfire which has a max hit of 50 if unprotected. His Poison, Shock, and Freeze breaths can be reduced to 10 damage with the Antidragon or Dragonfire Shield. The Antifire potion has NO effect against these 3 unique attacks, only the normal dragonfire. Thus, the player will be fully protected from the KBD's regular dragonfire with the shield and an Antifire, but his special breaths can still deal up to 10 damage.
don't forget the anti-dragon shields! Which can only be obtained during the Dragon Slayer quest. Lovely right?
Would KBD be difficult to put into the game?well, um it'll take some time lol