A is for Ark
The Ark is a mysterious Forerunner device from which all the Halos can be activated. At the end of Halo 2, Delta Halo is armed and then unexpectedly shut down, causing it to send a superluminal signal to all other Halos in the galaxy, putting them on standby mode. 343 Guilty Spark informs Miranda Keyes and Sergeant Johnson that the Halos must now be activated from "The Ark." So, where's The Ark? Rumor has it, its location is Earth, buried under the ground near New Mombasa. When the ship commanded by the Prophet of Regret jumped into slipspace from inside the Earth's atmosphere, it had a devastating effect on everything around it, caused the destruction of New Mombasa, and aided in unearthing The Ark... the huge structure glimpsed in the Halo 3 trailer.
B is for Blood Gulch
With Blood Gulch (or Coagulation as its been known since Halo 2) being one of the most fondly remembered multiplayer maps, we're thrilled to see its return in Halo 3, completely reimagined and sporting its newest moniker - Valhalla. While we know it packs the new Man Cannon, Spartan Laser and a few Mongoose to tool around in (See V, for more details), it remains to be seen whether all of the new additions and more asymettrical design will makes things even more hectic or ultimately harken back to the simpler, somewhat more focused Halo: Combat Evolved styling.
C is for Covenant
The Covenant will return. That's a given. What's more interesting is that we can say with some certainty that the Arbiter will be back too. He does, after all, have a substantial amount of story still left to tell. What's currently up in the air, however, is whether or not he'll be playable. Many weren't keen on the Arbiter levels in Halo 2 and felt they didn't get a big enough slice of Chief action. This time round Halo 3 will be mainly MC-led, but a piece of concept art (seen in a Halo 2 documentary) for the level shown in the announcement trailer clearly brandishes the production name for The Arbiter - 'Dervish' . This suggests old Arbi' will be playable - and maybe we can choose which side of the story we want to delve into this time.
D is for Drinol
The Drinol are Covenant bosses that didn't make it into Halo 2 - they should make an appearance in Halo 3. In the Halo 2 "Behind the Scenes" DVD, Drinol are described as huge beasts segregated from the other forces. Possessing phenomenal strength, they can smash enemy vehicles to pieces. In a recent update from Bungie themselves, with regards to what creatures they were working on, they simply said, "something big". Our bet? A Drinol Beast.