Complex sentence: In the year 172 of the Fourth Age, Findegil completed a fair copy of the "Thain's Book", the most complete copy of the "Red Book of Westmarch" wherein Bilbo Baggins, Frodo Baggins and Samwise Gamgee recorded their adventures.
Split into simpler sentences: In the year 172 of the Fourth Age, Findegil completed a fair copy of the "Thain's Book". The Thain's Book was itself the most complete copy of the "Red Book of Westmarch" wherein Bilbo Baggins, Frodo Baggins and Samwise Gamgee recorded their adventures.