This map is very, very interesting. If you fail level 4 (and it's rather hard to do so), the Marathon network resynchs and you're sent here to destroy Durandal's wireless transmitter in an effort to trap Hathor. This is also the first level of Chapter 1's failure timeline, meaning that if you can (or choose to) beat level 4, you never have to play this one.
This map is WAY too big and complex and such for me to give a walkthrough as I've done before, so I'll just go over some things I noticed. First of all, the platform trigger polys get VERY annoying. You'll spend a long time beating your head on your desk wondering how you open a specific door, only to find that it's magically opened when you stepped on some polygon in your quest. Also, I remember, Forrest, that you told me this map used to be an M2 map, and that the mapmaker (and I can't remember his name) retextured it. If you're wondering about the Lh'owon wind sound in the central area, that's why. I guess he must have forgotten to remove it, and it was an outdoor area in M2 form.
This map is, in spite of the many annoyances, pretty fun. The carnage is fast-paced (when you're not searching high and low for a switch), but Eternal's weapon system shows its flaws in this level at times when you just can't find a weapon anywhere but need to clear out a big room. Suggestions about the weapon system (I'll elaborate on these some time later): maybe, if you already had a weapon, picking up another one would just give you more ammo? Another suggestion is that you have actual ammo readouts - they'd make the game that much more enjoyable.