9. Serious Sam: The Second Encounter - Watcher
One of the finest arcade-style first-person shooters out there, Serious Sam: The Second Encounter was brilliant unadulterated carnage. It was what you wanted in a shooter of its time. It also houses one of the most unnerving easter eggs out there, and perhaps the creepiest rendering of a pair of eyeballs ever made.
In the last courtyard of the game, there is a single health pack sitting on its own in one of the corners. Collecting that pill causing a pair of eyeballs to spawn in the sky and stare at you in the weirdest way possible.
Naturally, if you are a normal human being, you'll shoot the eyeballs, and cause the watcher to spawn. A loud scream will sound and the kamikaze watcher will blow up, killing you instantly and leaving you without a single explanation.
It's a brilliant easter egg, because you can tell the developers had fun with it. From the bizarre rendering of the watcher, to the out of place eyes, the entire encounter it weird. And, for a player who stumbled upon this themselves, it is absolutely terrifying.