Faction Flag

Faction Flags are a category of Banner found in the hardmode Dungeon. These are placed by the game to communicate to the players where specific opponents will spawn. There are six types of faction flags: Not all of these will be found in one dungeon; on some occasions dungeons have been shown to have only three or four types, the most common of which appears to be Necromantic.
 * Necromantic Sign
 * Molten Legion Flag
 * Ragged Brotherhood Sigil
 * Rusted Company Standard
 * Diabolic Sigil
 * Marching Bones Banner

Trivia

 * If relocated, these will not start spawning faction enemies where they were placed. Faction Flags are currently purely visual.