Complex sentence: You are informed Illwhyrin (the witch player character killed in the first Witchaven) had a sister named Cirae-Argoth, one of the most powerful witches who ever existed, who now wants to avenge the death of Illwhyrin by killing your countrymen.
Split into simpler sentences: You are informed Illwhyrin (the witch player character killed in the first Witchaven) had a sister named Cirae-Argoth. Wicked Cirae-Argoth is one of the most powerful witches who ever existed and now she wants to avenge the death of Illwhyrin by killing your countrymen.