To fight this version you need to give the Nexus-keyed Mana Shard to the Ghost of Galaeral near the exit portal at the start of the keep. This summons a portal in the boss room at the very bottom that leads to Aerfalle's Sanctum.
A coolly amused voice seeps across your mind, "Vermin!"
A coolly amused voice seeps across your mind, "Don't you have anything better to do than bother me?"
A coolly amused voice seeps across your mind, "Disturb not your betters, child!"
A coolly amused voice seeps across your mind, "Gentlemen, could I trouble you to perform thy task?"
A coolly amused voice seeps across your mind, "A skillful attack, <Character Name>."
A coolly amused voice seeps across your mind, "Impressive, child. Your people do sometimes surprise me."
A coolly amused voice seeps across your mind, "A little help here, please."
A coolly amused voice seeps across your mind, "Your spellcraft is feeble, <Character Name>! Are you sure your calling is that of a mage?"
A coolly amused voice seeps across your mind, "Guards! To me!"
A coolly amused voice seeps across your mind, "Your so-called magics are no match for those of old Dericost!"
A coolly amused voice seeps across your mind, "Impressive, child. Your people do sometimes surprise me."
A coolly amused voice seeps across your mind, "Did that hurt thee, <Character Name>?"
A coolly amused voice seeps across your mind, "Perhaps I play too roughly with you fragile outlanders..."
A coolly amused voice seeps across your mind, "Is that the best you can do, my poor child?"
A coolly amused voice seeps across your mind, "Is that the extent of thy powers, child? I shall show thee true power!"
A coolly amused voice seeps across your mind, "Your so-called magics are no match for those of old Dericost!"
A coolly amused voice seeps across your mind, "I am one of the primogen of Dericost. I am not like mine countrymen you have fought. If you strike me down, I shall rise again!"
A coolly amused voice seeps across your mind, "That head will make a fine trophy. Which of you will next decorate my keep?"