Ancient Murals are Don't Starve Together objects, introduced in A New Reign. They are present in the Atrium Biome, appearing in bigger quantities near the Ancient Gateway. Their only purpose is to be inspected for a set of 5 different quotes for each character, giving away part of the history of the Ancient Civilization.
Upon reading a mural, the character will say their first quote. Reading the same mural will always give the same quote. In order to get the second one, the player must find another Mural, and so on. After getting the 5th inspection, the next one will cycle back to the first.
Depicts an old and underdeveloped civilization, composed of insect (or likely arthropod) people. They live in misery, suffering from hunger and fear. All before they discovered Nightmare Fuel.
Depicts a massively advanced civilization, with high technology and prosperity. All the problems seem to be solved. This appears to be after the Nightmare Fuel and the advent of the Thulecite and before their eventual destruction due to the overuse of the same fuel.
Trivia
The Ancient Murals and other elements present in the Atrium seem to be made of a material different from Thulecite.
The Atrium Ancient Statues are an exception to this, having their inside made of Thulecite. Although this may be a reference to the 4th Mural inspection.