King Black Dragon is the fiercest of all the dragons. So you got to be careful or he can badly burned you. It's wise to bring an Anti-Fire Shield so that you do not get badly burnt by the fire! This boss can be found in two places but there's only one of this boss in each place.
You can find this boss by going to 55 Wild(if you die you will lose your items) and by typing ::dragonbone or using the Boss Teleport System.
You can use any attack but it's wise to use Mage or Range, such as an royal sicle(u)or Assault rifle. Your Also going to need "Soulsplit". It's recommended to have your prayer "Soulsplit" level at 92 or higher. If it's not you're going to need a lot of prayer potinons. It's also recommended to bring an Anti-Fire Shiled so that you don't get badly burnt by his beast.
Since the King Black Dragon is a boss, it is included in the Soul System, converting all common and uncommon drops into souls. The King Black Dragon, along with all other NPC's in Dreamscape, drops the Infinity Key and immortal stone fragments