Sir Lancelot's Suit of Armour
Sir Lancelot's Suit of Armour is one of the two Murals in the Rise of the Tomb Raider: 20 Year Celebration DLC Blood Ties - the other one is Sir Reginald's Suit of Armour. It belongs to the Suits of Armour set and consists of a suit of armour with a shield and a sword. It is located in the Library.
Sir Lancelot can be considered one of the many Blood Ties Easter Eggs as his shield played an important role in Lara Croft Tomb Raider: Legend. The sword referenced in the description text is probably Excalibur form the same game.