The Moon Lord is considered the "final" boss in Terraria, and is based off of Cthulu's brother.
The Dungeon Guardian has the highest defense of any enemy in the game, topping off at 9999 defense.
Terraria has a large amount of references to other games, with some other games even referencing Terraria. A prime example of this could be that Terraria says to play Minecraft (however, this is only shown on the PC version) and Minecraft saying to check out Terraria.