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First you must complete the Vyraal hunt and receive the Dragon Scale. Give this to the Wyrm Philosopher (examine Windmill 10 at Cerobi Steppe) and he will hand you the Age-worn Key.

Now go to Destiny's March in the Sochen Cave Palace. Starting from the eastern room in the "diamond" shape, walk clockwise around it, opening only the right doors marked "Door Of Hours". Don't worry if you make a mistake, just leave the area to reset. If you do it right you'll hear a chime, and a message will tell you a door has opened.

You can now go through the Ascetic's Door on the western side of the chamber. Head down the passage, and use the Age-worn Key on the next door. This will grant you access to the Hell Wyrm's lair. It has around 9,000,000 (9 million) HP and absorbs Dark damage. Use Holy Lances and Excalibur to deal huge amounts of damage, and watch out for its Judgement attack - you can use White Masks to absorb this.

Sorry for sounding like a text book, but this is all the information you'll need to find it. I also have battle strategies if you need them.