Complex sentence: Many of the Eldar mourn the victory as bittersweet for the One Ring, despite their sacrifice, was not destroyed but kept by Isildur who later perished, losing the Ring in the Gladden Fields.
Split into simpler sentences: Many of the Eldar mourn the victory as bittersweet for, despite their sacrifice, the One Ringwas not destroyed. It was kept by Isildur who later perished, losing the Ring in the Gladden Fields.