The DOOM series has been known for it’s wide variety of easter eggs over the years. This one is an absolute pearler. When DOOM’s soundtrack is put through a spectrogram, it shows the numbers 666 quite clearly. These numbers are considered to be the number of the beast and is associated mainly with the devil. DOOM’s composer Mick Gordon has himself confirmed the presence of demonic symbols in the game’s soundtrack.
Some of the other easter eggs include retro maps that mimic the game’s original release as well as the iconic scene from Terminator 2 which shows you getting eaten up by molten lava.