In Halo 3, the IWHBYD (I Would Have Been Your Daddy) Skull is located on the level The Covenant (Level). For this skull, you may be on difficulty levels: Normal, Heroic, and Legendary to acquire the skull.
In order to get the IWHBYD skull, you must complete the entire mission until you reach the final battle scene, where the Prophet of Truth is planning on lighting all seven rings. Once you reach the bridge, battle your way across with the Arbiter and Flood, neutralize all Covenant, watch the cinematic, and kill all the flood. Once the bridge is devoid of enemies, you will have to literally jump through the 7 rings in a complex order. Once you have completed this, the IWHBYD skull will appear on Truth's podium.
There are two sequences that both work. One contains 18 jumps, while the other has only 7. Both will produce the IWHBYD skull, the difference between them is that one obviously takes longer. The purpose of the longer code is to provide a "backup" in case you might make a mistake or perhaps the shorter simply doesn't work.
Note: Each number corresponds to the ring you must jump through. The ring at the elevator is 1. The ring next to Truth is 7. The ring with red on it is 4.
Sequence order:
>Shorter order: 4,6,5,4,5,3,4
>Longer order: 1,3,4,6,7,5,4,6,5,4,5,3,4,5,4,5,3,4
Once you have completed either sequence, the skull will appear at the start at the bridge at the end. The Halo theme will play as you get the skull.
NOTE: You must complete the whole level.
The IWHBYD Skull unlocks the extra dialog in the game, which usually comes from the UNSC Marines and Elites, but some ridiculous dialogue does come from enemy forces with this skull on. For example if in a group of two Brutes one is killed, the remaining Brute has a chance to say "You killed my lover!" or such as the Flipyap quote in which one Grunt is killed and another screams "You killed Flipyap! Or Yapflip, was he... It was Flipyap... No, Flipyap was his brother. Don't tell me I don't know Yapflip! Flipyap and I went to Nipple Academy together, and, eh, now he's dead...". Other notable quotes from enemy infrantry and Brutes include "I like chicked neggets!", and "Sorry, we don't take master card".
Notable IWHBYD effects from allies and other AI/NPC's include "That one called you fat Chief!" (exclaimed by Sergeant Johnson), "Chief, can we be friends?" (from Marines and occasionally Elites), and if one shoots the Monitor with the IWHBYD skull activated, he will exclaim remarks such as:
"That hurts my feelings, oh my God I have feelings! I'm a real boy!"
"You're pissing me off! Oh my! I didn't know I had cussing in my programming"
"Are you always a pain in the ass?"
Trivia
The name of the skull is based on a statement uttered by Sergeant Johnson in the first Halo: "I would have been your daddy, but a dog beat me over the fence!". It is also the name of the first chapter of the campaign level Assault on the Control Room on Halo: Combat Evolved.
Each of the holographic Halos at the end of The Covenant sounds a different tone of the E Dorian scale when you stand near it. The sequence, if you pay attention, plays part of the Halo theme song.