Assembly

Assembly is the official name for the map that was announced in the Bungie Weekly Update (8/22/2008). It was originally planned to be in the Legendary Map Pack, but was replaced by Blackout due to the fear that it wouldn't be finished on time. At the current time there is no known release date for this map.

Design
The map, as seen by fans at PAX 2008, is based inside a Covenant Scarab factory. When looking overhead, Scarabs can be seen moving by some kind of automated gravity beam. As such, the area outside the level is immense.

The map itself is bowl shaped, sloping toward the center. This bottom floor is dotted with pillars and walls for cover. It has an outer rim with four bridges that connect to the two-story center structure, and has small structures on the rim intended as high ground. It appears to be symmetrical. Line of sight is generally open, with mid-range combat being optimal when on the second level. However, within the center structure and around the pillars that dot the bottom level, close combat is possible.

The map is focused on competitive playing, as it is multi-story arena-style map. It features two "somewhat enclosed" bases, with one-way mirrors on the second floor (like those in interrogation rooms), where players will be able to look out, but players outside will be unable to see in.

Mystery
Before Assembly was removed from the Legendary Map pack, it was stated that one of the maps would expand upon the Scarab-Hunter mystery. In a recent Weekly Update, it was stated that, "One base of this open-air arena could end up getting called 'Energy side' and another call-out could end up 'Hunter side'", so it's possible that this was the map they were referring to. Adding to this mystery, Luke Smith stated on a thread on the Bungie forums that "the name Assembly is extremely appropriate." This would refer the fact that Scarabs are assembled within the facility. The Scarabs are carried along an assembly line, with the playable area of the map being one of the machines that constructs the scarabs.

Trivia

 * In the images, fog can be seen in the distance if you are far enough away from a structure. Similar fog can be seen on the Halo 2 level High Charity near in the parts with Flood.
 * The former name of the map, Purple Reign, could of been a homonym with the TNT firework "purple rain", it could also be linked as a tongue in cheek reference to the song "Purple Rain" from the artist Prince, but it is most likely referring to the dominance of the color purple on the map.
 * A screenshot has been found in a NY Times blog that some thought was a Bungie employee working on a 3D model of Purple Reign. However, the structure is, in fact, a structure from High Charity and can be found on the Halo 2 levels Gravemind and High Charity. This would also fall into the scheme of the in-production name resembling the look of the level, while as the real name represents gameplay.
 * Assembly was first announced to be part of the Heroic Map Pack (while at that time it was called Purple Reign instead), which was soon replaced by Foundry, and then announced to be part of the Legendary Map Pack, but replaced by Blackout, which shows that this map project has been going on for quite a while in Bungie. This long delay of release may possibly mean that the map has a very complex design layout for gameplay.
 * Although not a true screenshot, a pen rendering of the map was featured in the Bungie Weekly Update on July 25, 2008.
 * This map may be released with Space Camp in the next map pack.
 * Players who made it to the PAX gamers convention in Seattle WA, have played Assembly before it comes out as DLC on Xbox Live.
 * The map was originally intended for Slayer based games, but objective games are described as playable and frantic.
 * Assembly is the second Covenant-themed map in Halo 3, the other being Snowbound.
 * A recent Bungie Weekly Update has released several screenshots for those who are unable to reach PAX.