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

The spawns themselves are controlled by the walls of the dungeon, not the flags. For instance, standard brick walls will spawn Necromancers and Blue Armored Bones, while slab brick walls will spawn Ragged Casters and Rusty Armored Bones.

Trivia

 * If relocated, these will not start spawning faction enemies where they were placed, and removing them won't prevent the enemies they correspond to from spawning. Faction Flags are purely visual.