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Played it again. It turned out pretty well – I only failed to capture one mine, and my outposts weren't captured by the Necromancers, so I spent the time improving my skills. Then, the Necromancers finally captured one of my outposts, I recaptured it, and Biara arrived. I ran with her along parallel paths to the castle, and then easily defeated her. Mission completed. By the way, I realized I had a licensed copy, so I still don't understand why it was glitching before.