Complex sentence: Robert Bruce, the 2nd Lord of Annandale, also known as Robert de Brus, was the third of his name in the Scottish Bruce family and an ancestor of Robert I of Scotland most commonly known as "Robert the Bruce".
Split into simpler sentences: Robert Bruce, the 2nd Lord of Annandale, also known as Robert de Brus, was the third of his name in the Scottish Bruce family. He was an ancestor of Robert I of Scotland who was most commonly known as "Robert the Bruce".