I did it at 18, but you could do it as soon as you have hide and sneak; just don't get seen by the assassin. If you camp there, someone will kill him and the mask will be left to rot. But be careful; monks like to use FD when they pull, which can set him onto you, so don't camp right under the Ass/Sup room and run away if the party there look like they are about to wipe before killing him.
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Mentathiel Rogue and haunter of level-inappropriate dungeons
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