Dagannoth Kings are a group of RS gold three monsters that can be counted as one boss. They are able to be found on the Waterbirth Island 6th level of the dungeon. Each has an enlisted level of 331 and is a representation of one of the three combat styles that are: Ranged, Melee and Mage. Additionally, each is able to drop a an exclusive ring, such as Berserkers, Seers, or an Archers ring.
Since getting there is difficult and could be the most difficult part of your battle against your boss, it is best to go with someone who knows the best way to go or consult YouTube route guides prior to going. Every boss has the chance of dropping an Dragon Axe.
The angel-like creature is in fact the head of Saradomin forces that fight in God-Wars Dungeon. Her combat level is 596, which means she is a mid-game boss available to all players who are able to gain access to this area and take out 40 Saradomin creatures that open the door into her cavern. The boss is usually the target of being killed for God Sword pieces and Armadyl Crossbow which can be very costly. The battle cheap OSRS gold against Commander Zilyana is easily repeated, it is easy to farm her, especially in close proximity.