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When your character reaches level 71, it is eligible to attempt the Master Quest.
If you successfully complete the Master Quest, you will unlock the next secret character and go back to level 1. If you fail, you must start back at level 1, and try again. If you are in a Guild, you and your Guild will receive Guild Points for your attempt. Refer to the Classes page for how many guild points are rewarded. If your character is Hardcore, you will receive double the listed amount of Guild Points for a pass or fail.
You will not lose any items or proficiencies by attempting the Master Quest.
tyvm man