Complex sentence: The Lord High Commissioners were appointed from 1603 until 1707; the Act of Union 1707, which merged the Parliament of Scotland and the Parliament of England to create the Parliament of Great Britain, rendered the post redundant.
Split into simpler sentences: The Lord High Commissioners were appointed from 1603 until 1707. The Act of Union 1707, which merged the Parliament of Scotland and the Parliament of England to create the Parliament of Great Britain, rendered the post redundant.