Be careful in the lair, as the other two kings can hurt you if you are not using their specific protection prayer.
The Dagannoth prime casts what's like water wave on steroids. Simply turn on protect from magic and he won't be able to deal any damage to you. Also, he seems to hit the people who are attacking him with melee the most - or people who are right next to him. So it is a good idea to range. The only thing you have to be wary of is Rex running into you and damaging you badly. Also, his Magic can hit up to 640, and it will hit anyone within 3 spaces of its destination, and anyone in the path of the spell.
A legendary dagannoth king, rumoured to fly on the North winds.
Clue scroll (level-3), Clue scroll (level-4), Dragon med helm, Shield left half, Dragon hatchet, Mud staff, Seers' ring
Dagannoth bones (100%), Dagannoth hide (100%), Coins (976-1000), Steel kiteshield, Ranarr weed, Herb seeds, noted Pure essence (134-194), Oyster pearl, Air talisman, Air battlestaff, Earth talisman, Earth staff, Earth battlestaff, noted Battlestaff (9-10), Water talisman, Shark (5), Uncut gems, Rune kiteshield, Skeletal top, Skeletal platelegs, Farseer helm, Air rune (155), Earth rune (67), Death rune (60-77), Blood rune (97), Mud rune (32), Fremennik helm