Complex sentence: In a final confrontation near Mount Doom Sauron killed both Gil-galad and Elendil, but was also cast down himself by Isildur, who took the remains of his father's broken sword Narsil, and used it to cut the One Ring from Sauron's hand.
Split into simpler sentences: In a final confrontation near Mount Doom Sauron killed both Gil-galad and Elendil, but was also cast down himself. Elendil's son, Isildur, took the remains of his father's broken sword Narsil, and used it to cut the One Ring from Sauron's hand.