Warning: Do not stray from the path! The lava here will kill you very, very quickly, and the cliffs do not have invisible walls. In other words, the only Ouch!! you will get when moving off the edge of the cliff is the one when you hit the bottom and die.
From the beginning point, in the center of the first square:
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Answer the first riddle. Answer is
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7000000
S
Answer the second riddle:
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Glacier
S E W W E S E
Then follow the narrow path (see warning above) over a few lava pits. Use Restoration as needed. You will have to fight two fixed battles before reaching the Dragon's Grave.
First fight 25 Fire Guardians.
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This is an easy fight if you have been through Inferno. Just use Close Combat to bring them into melee range.
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Use Close Combat and Hesitation.
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The previously locked door in the far SE corner of the basement of the Castle Tower (aka Abandoned Tower) south of Castle Cranbourgh.
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Enter the Gateway to the Planes. Once you have done that, you can dump it by visiting a camp (where you can pick up your seventh character again) or by taking the road wardens, who do not accept non-player characters.

By the way, like all wandering creatures, the dragon will occasionally turn on you. Keep an eye on its mood and cast Camaraderie as needed to calm it down.