Complex sentence: Geralt faces off against Eredin in a brutal fight and upon strikking the mortal blow on Eredin, he "reveals" that Avallac'h has Ciri and that he lied to them both.
Split into simpler sentences: Geralt faces off against Eredin in a brutal fight and emerges victorious. In his dying breath, Eredin "reveals" that Avallac'h has Ciri and that he lied to them both.