Complex sentence: Lady Diabolyn was a stepsister to Queen Sarana whom she always considered weak and unfit to rule; to gain her "rightful" throne, she learned dark magic and allied herself with the demonic Spectre.
Split into simpler sentences: Lady Diabolyn was a stepsister to Queen Sarana whom she always considered weak and unfit to rule. To gain her "rightful" throne, she learned dark magic and allied herself with the demonic Spectre.