Complex sentence: Nardole is introduced in 2015 Christmas specisl "The Husbands of River Song" was hired by Proffessor River Song as a servant on Mendorax Dellora on Christmas Day 5343.
Split into simpler sentences: Nardole is introduced in 2015 Christmas special "The Husbands of River Song". He was hired by Professor River Song as a servant on Mendorax Dellora on Christmas Day 5343.