The Elder Sign is a special sign used for spawning Cthulhu.
the elder sign looks like a yellow sign with a pentagram in the middle. Only one spawns naturally per world on the r'lyeh tower, which spawns in the middle of the Atlantis Biome.
The elder sign can only be broken with a diamond sword or higher.
It is possible in creative mode or through cheats to obtain this sign. In creative mode, one simply has to use the "pick block" function. Once obtained, one can spawn a Cthulhu in the overworld.