An Eternity code is the code which is placed upon the C Cube. It is a code based upon a new language so the computer has no frame of refence to decode it and is meant to take eternity to crack.The C Cube's full potential will only be reached if Artemis Fowl uses his voice to deactivate the code.
Spiro attempts to crack the code, but realizes that he cannot. So, he tries to force it out of Artemis, but fails.
If you want the font and the instructions to put it in, go to this link .