One of the most difficult skulls - or easter eggs - to find in Halo 3 is known as the IWHBYD skull. This stands for I Would Have Been Your Daddy. The skull takes a great deal of work to find, but is well worth it!
There are many things to keep in mind when going for this skull.
1) Start from the VERY BEGINNING of the level called The Covenant. Not just the chapter - the entire level. Make sure you're playing on any difficulty EXCEPT easy. Normal is obviously the one to try for :)
2) Go all the way to the end of the level, meaning you kill the Prophet and then kill ALL of the flood. You must kill all of them so that the "danger" music stops playing.
At this point, you'll have nice quiet background music. When you walk past each ring, you'll hear it humming. Each one hums a different note! What you are about to do is "play" the Halo theme.
3) Number the rings in your mind. Ring #1 is by the elevator, Ring #7 is by where the Prophet was standing. Ring #4 is the one with a red area on it.
4) You're going to jump through the following rings, in order. If you screw up, just start again with the first ring in the sequence again. You do NOT need to die or reset or anything else. Just start again. The sequence is:
5 4 6 5 4 5 3 4
Once you do that, all the rings will begin flashing immediately. That is how you know you've done it properly. Then go down to the end where the Prophet was and pick up the skull.
5) FINISH THE LEVEL holding the skull in your hand, then save.
This does NOT unlock an achievement. However, if you have now "completed the set" of all the skulls, you do unlock new armor.
On its own, this skill gives you a whole set of new dialogue options. So as you go through playing the game you'll hear funny new comments from the marines, elites, grunts, you name it. It's pretty interesting.
Note that it took us SEVERAL tries to get this to work. The two main problems we had were
1) We didn't start from scratch. You really need to start from the beginning of the entire chapter and play through.
2) We didn't kill ALL the flood at the end. There were still red dots on the map and danger music playing. We couldn't even hear the notes the rings were making! Make sure the danger music is NOT playing when you attempt this. As you walk near each ring, you should be able to hear its humming noise in the note it plays.
I don't mean the rings PLAY a note when you jump through them. I mean that each ring naturally EMITS a note / hum when you are near it and you can hear the notes as you go walking past them.