Halo 3/Denouement

Although this section of the game is not a level, it is an important cinematic sequence in terms of canonical significance.

Easy Ending
Some unknown time after the Ark's destruction, Lord Hood, the Arbiter, and several marines are shown at the dedication ceremony of a war memorial in honor of those who fell during the war against the Covenant. After delivering a short speech, several marines fire their rifles in ceremony.

Lord Hood then has a short conversation with the Arbiter, during which he states that the Master Chief died in the explosion. The two shake hands and part ways. The Arbiter then returns to the Shadow of Intent and leads the Elite ships to the Shanghelli homeworld.

The camera then pans over the monument to show the numbers "117" hastily engraved on it, as a way of honoring the Master Chief. The screen fades, and the credits begin.

Normal/Heroic Ending
After the credits, if you completed the game on Normal or higher difficulty, another scene is shown from the Forward unto Dawn.

The Master Chief regains consciousness inside the rear half of the ship and discovers that they had only been partially successful in escaping the destruction of the Ark. Cortana drops a distress beacon, but warns that it would take months or even years before they are found. The Master Chief then makes his way to a cryostasis chamber, stores his assault rifle in a nearby gun rack, and enters stasis.

Legendary Ending
If you completed the game on Legendary difficulty, an additional scene is played.

After the Master Chief goes into cryostasis, the severed section of the Dawn is shown drifting towards an unknown planet which appears to be covered almost entirely by cities, and the cutscene ends.