Very Difficult - Endgame: The Philidor position is one of the key positions in which the stronger side wins in Rook + Bishop vs. Rook. It's useful to know about it in case you get that endgame in your tournament practice.
Difficult - Study: A very beautiful 1924 chess study by Troitsky. In this short video, Chess Queen Alexandra Kosteniuk shows you how to solve it to the end, you will enjoy it for sure.