Reading the book from your inventory will cast the spell required to turn Amdusias human again, so (after a conversation) you can fight/defeat him. You can read the book wherever you wish; volume 1 contains the spell Amdusias used (though you don't need to read it), while the second volume, from the librarian, contains the counter-spell.
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BTW, if you edit duplicate post in the other forum, there is a 'Delete Post' button beside the 'Change Post' and preview buttons, which will remove the topic, as well (as long as nobody replies to it). This will not drop your post count.
