|Location||Waterbirth island dungeon|
NotesBe careful in the lair, as the other two kings can hurt you if you are not using their specific protection prayer.
TacticsDagannoth Rex cannot be hit by melee or range, so mage him from the edge of the moat. Rex likes to move around a lot, and his huge tail will swipe out at you and deal up to 260 damage if you get too close. Also, when he swipes, damage will be dealt to multiple players if you're standing together, so spread out if you're there with a team. If you see Rex coming for you, break off the attack, get back and wait for him to attack someone else, and then recommence attacking.
You can solo kill or dual kill the Rex by bringing a high level spell such as Fire wave, Magic dart, or an ice spell and maging Rex from the south-east corner of the island next to the seaweed. Be sure to bring plenty of prayer potions, food, and a teleport out. Bring Guthan armour if you can afford it for its healing properties.