Old School RuneScape: How To Solo Dagannoth Rex

thegamer.com:

The Waterbirth Island Dungeon in Old School RuneScape houses many fearsome creatures - most notably, it is home to the three Dagannoth Kings. Each uses a different combat style - one melee, one ranged, and one magic -, and they all weigh in at a massive combat level of 303. Most players will forego two of the Kings and instead set their sights on Dagannoth Rex.

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You should not take the challenge of soloing a Dagannoth King lightly, although Dagannoth Rex is the easiest to solo of the three. He also has the potential to drop an impressive array of rare and valuable rewards, such as the Berserker Ring and the coveted Dragon Axe. Each kill guarantees you some Dagannoth Bones at least, which fetch a modest sum at the Grand Exchange.

Dagannoth Rex, alongside the other Dagannoth Kings, is located at sublevel six of the Waterbirth Island Dungeon. There are no formal requirements to traverse the dungeon itself, although unlocking the ability to travel to the island is a different story; if you want to travel to Waterbirth Island, you'll need to complete The Fremmenik Trials quest.

This will unlock the ability to travel to Waterbirth Island by boat when speaking to Jarvald in Rellekka. If you want a faster way of accessing the island, you can instead use the Waterbirth Teleport spell from the Lunar Spellbook - given that you have it unlocked. Since obtaining this spellbook requires The Fremmenik Trials to be completed anyway, there is no way to access Waterbirth Island without completing this quest.

Sublevel six, home of the three Dagannoth Kings, is the lowest level of the dungeon - keep heading down the ladders you find across the dungeon and you'll find it without much