well what i did. was cast prot evil 10'. then i cast gate and watched the pit fiend make the dragon run!. the dragon had a yellow circle. i then cast a bunch of spells and by the time he returned his morale he was dead.

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Master Of Sorcery... Slave To Darkness

I don't use demons for attacking. I use them as free XP for my party. If your Vorpal Sword had done the trick, you would have seen some sort of white spirit rise from his body