Complex sentence: "Symbiosis" was filmed after "Skin of Evil", which featured the death of Tasha Yar; this was the final filmed appearance of Denise Crosby as Yar until she reappeared in "Yesterday's Enterprise" due to a change in the timeline.
Split into simpler sentences: Symbiosis was filmed after "Skin of Evil", which featured the death of Tasha Yar. However, this was the final filmed appearance of Denise Crosby as Yar until she reappeared in "Yesterday's Enterprise" due to a change in the timeline.