Half-Life: Action Half-Life Easter Egg - The Secret Hallway (and so Much More)

This egg is probably one of the best eggs ever hidden in a game, because it's just so involved and cool, involving a ton of work and even time travel...all of which is completely detached from the main level. Not to mention, it explains the strange egg in the 'No Credit' level, and fills in much more of the story of Hgrethedelon, the giant eyeball that you encountered in the aforementioned secret.

Let's walk through this step by step. First and foremost, you'll need to enter the level 'Endless Rain 2'. The filename is "ahl_er2.bsp."

1. In the lobby where the music is playing is a booth with a large glass window and a chair. Break the window, and pull the chair outside.
2. Guide the chair downstairs (hopefully you haven't broken the glass floor!), and to the bridge that runs across the aquarium. Look down to make sure you know what side of the bridge leads directly down into the aquarium, and push the chair off that side. It will float to the bottom.
3. Jump in after it, and you'll be transported to one of the locked sleeping chambers. Someone in here apparently didn't have a good experience with Hgrethedelon...
4. Shoot the dynamite on the other door to access the hallway. Move across to the green lit door.
5. Go down the stairs, and enter the warehouse.
6. In the corner to the right of where you came in is a blue button on the wall behind a number of crates. If you have AHL, stand against a crate and jump to climb it. If you're playing the map outside of AHL, you'll need noclip. When you get back there, press it, and you'll hear something drop.
7. Climb onto the crates (again, only with AHL), and carefully jump across to the other side of where you came in. There is a large crate in a pit, and you can cross it to reach a door.
8. Inside the door is a small control room with a floor vent. Drop down into the vent, and break the series of grates until you reach a red-floored room.
9. The room will light up - it's filled with bodies, and eventually blood will start pouring in. Scan the ceiling above you - there's a bright-white lit grate. Wait until the blood is high enough, and swim to the new grate.
10. In the new chamber, stand on the floor hatch to drop down. Fall through the next one, and you'll end up in a metal hallway. Go through the door at the far end.
11. You're now in the Center Of Reality, which contains a series of puzzle locks, and eventually leads to back the secret in No Credit. For now, worry about the puzzle locks. Around the building are three types of doors - blue, red, and yellow. To the right of where you entered the room with the sign is a small, blue door. Open it, and press the yellow button behind it.
12. Now, take the blue elevator (activated by a floor switch), and press the button at the top. While you're here, aim through the blue force field. You can see a grate with a thick, metal covering over it. Shoot at the cover, aiming at the bar closest to the far wall. Eventually, it will break. Now, go through the grate near you, and drop into the lower floor again.
13. Go back into the starting room, where there's a movable canister. Push it into the room with the sign. To the left of the sign is a small, horizontal bar. Run and jump onto the canister, then the bar, then the top of the sign. Above the sign is a grate. Enter it.
14. Climb up the ladder through the now-unlocked grate, and into the laser room. Be careful not to get hit, because you will take damage. Press the third yellow button, then make your way back to the room with the sign.
15. Go up the blue elevator, and into the yellow door. At the end of this small room is the first red button. Press it, and carefully make your way back to the bottom floor.
16. Now, open the large yellow door across from the elevator. Flip the switch to turn off the deadly red lasers in the turret room (which you'll find later).
17. Climb back up into the laser room again, and leave through the door. At the end of this transparent hallway is another red button. Go through the red door near you, and then move forward, aiming for the vent on the structure below.
18. Break the vent, and move forward. Don't go into the large room yet. Inch around the corner - you'll see a turret gun in a slit; destroy it. Then, run across to the unusable elevator, and do the same thing - peek around the corner, and destroy the gun. Then move through the door between the guns.
19. At the other end of this circular room is a series of doors. The curfew guy tells you, "Welcome to Hgrethedelon." The last door opens to reveal a giant, metal (and somewhat dysfunctional) Hgrethedelon, which is shooting at you. Move back into the main, circular chamber.
20. Bang around the walls to the right of where you first entered; there's a breakaway panel. Hit the square box inside to activate the hatch behind the grate in the middle of the room.
21. Drop through the hatch, and you'll be in a red room. Around the other side is a skeleton holding a device. Use the device, and the Time Shed will appear behind you. The Time Shed will take you back in time 200 years to 1999.
22. Once you've pressed the button in the Time Shed, you'll warp back in time and appear underwater. Swim out, and up. Across from where you are now is another pool. Swim down, through the tunnel, and up.
23. Break the ceiling, and swim upwards. You are now in familiar territory - the secret of No Credit, before any player has killed Hgrethedelon. However, do not kill Hgrethedelon this time around, as this secret becomes incompletable. Swim back down to where the Time Shed is.
24. You need to get your gun ready, so hide in the Time Shed until you are ready. Outside is a very strong current, pulling you towards 'Bitey', which will trap and eat you. Carefully swim against the current, looking up. In the ceiling right in front of Bitey is a white, glowing light. Shoot it until it turns dark. You'll now receive a message that Hgrethedelon is damaged, and has sealed itself off to repair itself. You have thus prevented No Credit from happening - one can not destroy Hgrethedelon; anyone who falls down that elevator shaft will simply rot there.
25. Return to the Time Shed and go back to 2199 by way of the button... however, times have changed. You're not in the Center of Reality, but on an island floating in a black abyss. (Incidentally, the Time Shed is still there... but invisible. The switch to take you back is against the cliff wall... just feel around, you'll find it. Not that you need to.)
26. Extending from the island is a breakaway stretch of land. You'll soon find out that looking up at the island is an enormous Hgrethedelon. Fall into its pupil... and you will fall into a black pit, and drown. The end.

Gotta love interconnected Easter Eggs that tell a story! Unfortunately, this is only one of two eggs that relate to the Hgrethedelon saga. However, this is NOT the most expansive easter egg in one of Hondo's AHL levels... that is in the level "5 AM", and he strictly forbids the solution to that one from being revealed.