The Mysterious Tablet is a decorative NPC found at the entrance to the dungeon after defeating the Golem. It never moves, cannot be destroyed or damaged, nor can it cause damage. When the Lunatic Cultist is summoned, the Tablet is destroyed until the Cultists respawn.
If a player has the Chlorophyte Armor equipped, the Leaf Crystal may attempt to fire at the Tablet.
It seems to depict the Moon Lord's face in the center.