Easter eggs have always been a hallmark of Fang Cheng Studio — one simply cannot miss out on them.
Wang had once heard from Monkey on the down-low that their boss, Fang Cheng, was a maniac for Easter eggs. He couldn't bear the thought of their games not being stuffed with Easter eggs, to the point that even the in-house planners would often stumble upon new ones while playing.
If you haven't encountered any Easter eggs while playing the game, then it means you haven't been playing attentively and thoroughly enough. As a penalty, you're sentenced to start over and play again.
Now, looking back, "Dark Builder" probably foreshadowed the "Shadow Invasion" event all along, with the solution hinted at early on; it all depended on how closely players were willing to look.
Luckily, I knew Fang Cheng well enough to notice this.