The Easter Egg is found at the end of a long secret corridor on Happy Happy, Carnage Carnage. If you examine it you'll learn that it's edible. But should you eat it? Well we'll come to that later.
The Easter Egg has an interesting history. The Official Hint Book makes no reference to the Egg or its purpose even though its secret location is revealed in the map of Happy Happy, Carnage Carnage. In the Nov '93 edition of Inside Mac Games magazine Tuncer Deniz wrote a Strategy and Tactics article for Pathways Into Darkness. In the walk-through section for Happy Happy, Carnage Carnage the following line appeared:
By the way, there is a hidden room on this level that no one until now knows about. It's on the north wall, look for it!
Wait a minute! On the north wall? It's on the south wall! No wonder nobody could find it. Thanks Tuncer! ;-)
Neither Pou Lee's "Pathways Into Darkness Basic Survival Guide" or David Reevely's "Pathways into Darkness FAQ Sheet/Walkthrough" make any reference to the Egg or its secret location.
Russ Taylor's Pathways point totals mini-spoiler which dates from Feb 5, 1994 does however reveal that you'll find a "nifty treasure" in a secret area on Happy Happy, Carnage Carnage. No other details are given though.
A year after Pathways Into Darkness was released Inside Mac Games magazine (Sept '94) published a letter from David Knauer in their Hints, Tips, & Tricks section which provided some details about the Easter Egg. Not just where it could be found but more importantly at what point it could be safely eaten. Here's the full text:
Pathways Into Darkness
There is an "Easter Egg" on "Happy Happy, Carnage Carnage that teleports you to the last level if you eat it. To get it, go to the southwest corner of "Happy Happy, Carnage Carnage" go along the wall till you go through the wall and when you go through the wall there will be a path go through the path until your in a narrow corridor with 3 teleporters but don't go in the teleporters. At the end of the narrow corridor there will be the "Easter Egg." Double Click it to eat it.
Before you eat it here's a tip: Go up to "The Labyrinth." At the southeast corner of that floor there's a ladder that goes down to a level that leads to "I'd Rather be Surfing." Go down the ladder to "I'd Rather Be Surfing." When you go down to the level go forward until you see a Sentinel sitting on the ground. Sentinels are immune to everything except the Green Crystal. Kill the Sentinel with the Green Crystal, then go forward until you see two doors, one on each side. Open those two doors. After you open them then it's alright to eat the "Easter Egg."
But after you kill the demon on the last level there will be the "Alien Gemstone." When you pick up the Gemstone it will start to suck your life away. To solve this problem you need the Lead Box. To get the Lead Box go to "Warning: Earthquake Zone." It will be there. But there lies "The Giant Purple Mutant Hellbeast" You can't kill him. Go around him to get the Lead Box. There will be a group of Ghasts (Earthquake causing monsters) but just shoot them. After you get the box then eat the "Easter Egg". Then when you beat the last level and get the Gemstone, just put it in the Lead Box and it won't suck your life away.
At the beginning of "Warning: Earthquake Zone" there will be green oozes. You can't kill them. But poison yourself with the Brown Potion. When your poisoned they will avoid you. On your way to "Warning: Earthquake Zone" you will come across a suffocation room with a dead body. To get past it put on the Red Cloak from "Happy Happy, Carnage Carnage" then rest till the doors open. Remember The "Alien Gemstone" is only chance of you getting out.
Remember to get the Nuclear Device and the Radio Beacon. The Radio Beacon is on the level "Need a Light?". And some guy named Steven changed the code for the Nuclear Device from "2870334" to "6580334". - David Knauer via America Online IMG: Hints, Tips, & Tricks
So bascially it is only safe to eat the Easter Egg after you get the Lead Box on Warning: Earthquake Zone. It's in the same room as Barney & Co.
Eating the Egg takes you directly to the last level where you have to face the twelve waves of monsters.
But what happens if you face the twelve waves first AND then eat the Egg? Well I'll let you discover that one out for yourself. Good luck... you'll need it. ;-)
Curious to know what happens when you finish the last level and then eat the Easter Egg?
Earthtopus <firstname.lastname@example.org> points out in this Story forum post that it has been documented in HB's Pathways Into Darkness Walkthrough in a Bonus Update section.
Here's the important piece from HB's walkthrough and I quote:
Once you kill all the monsters, you can kill the demon again and get another Alien Gemstone. They both drain your health and they both fit in the box. You don't get another 5 points, but you still get its treasure value.
We don't have the bomb any more, and it's disappeared from the floor, but the game knows we already planted it so we can teleport back out and win when we leave the pyramid.
The one caveat? If you haven't reached I'd Rather Be Surfing before eating the egg, even if you beat all the monsters and teleport out there's a locked door between you and the surface that can only be opened from the side you pass in the normal game progression.
Finishing the game after using the Easter Egg boosts my final score to 83- 2 points for picking up the Easter Egg, and 1 extra point for having more than $900K in treasure (the second gemstone puts us over the top).
I don't believe this has ever been documented on the internet before- pid.bungie.org is characteristically coy about it.
Hmmm... well it was also mentioned in this post on comp.sys.mac.games back in Jan 1994.