Complex sentence: The eldest legitimate son of Archibald Douglas, 3rd Earl of Douglas and Joanna de Moravia of Bothwell, he was born either at Threave Castle or at Bothwell Castle c.1372 and was known as the "Master of Douglas" until his accession.
Split into simpler sentences: The eldest legitimate son of Archibald Douglas, 3rd Earl of Douglas and Joanna de Moravia of Bothwell. He was born either at Threave Castle or at Bothwell Castle c.1372 and was known as the "Master of Douglas" until his accession.