Where can it be found

Waterbirth island dungeon, lowest level.

Tactic

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.

Examine Message

A legendary dagannoth king, rumoured to fly on the North winds.

Notes

Be careful in the lair, as the other two kings can hurt you if you are not using their specific protection prayer.