Complex sentence: Isildur claimed the Ring as an heirloom for his line, but when he was later ambushed and killed by the Orcs, the Ring was lost in the River Anduin at Gladden Fields.
Split into simpler sentences: Isildur claimed the Ring as an heirloom for his line. When Isildur was later ambushed and killed by the Orcs, the Ring was lost in the River Anduin at Gladden Fields.