Actually, Adventure, the Atari 2600 game that introduced video game Easter eggs to the popular imagination, among other things, now has its second sequel. Both were programmed by Byte Knight over on the Atari Age forums, and the latest, Evil Magician Returns II got an update on Thursday delivering a third level to the game.
It's the sequel to 2009's Evil Magician Returns, which references the unseen antagonist who had stolen and hidden the chalice in the original game. In Evil Magician Returns II you'll have to overcome both him and his vile apprentice, as they both personally guard the chalice. It's got keys, mazes, flickering grottoes and the bat. No word if it has the Dot and a developer credit Easter egg, too.
Update: Byte Knight replies to Kotaku: "You can't have an Adventure game without an Easter Egg. After all, it was the first game to feature one!"