English Grammar

Question

Answer key shows that the underlined words have the error, but it doesn't explain why. I don't know what's wrong with the underlined words, please explain. Or is it the semicolon? Does it contain an error? Or is my answer key wrong? I have asked opinions from others and they have different answers on this.

Alexander the Great was at the height of power when he died; thus are the mighty lain in low.

Answer 1

The answer here may be that he was in power up until he died, as he would no longer have power when he died.