A must have final expansion if you liked Galactic Civilizations II, and Dark Avatar.
Once again there's a campaign, but it's not worth the time to play, so let's go right into the additions and tweaks for the standard game mode. First of all there's a new Galaxy size for you to play with, "Immense" which is a step above "Gigantic". If you want an epic sized experience, then Immense is definitely for you. There's also of course the Custom pre-made Galactic Maps (such as the Milky Way which was introduced in Dark Avatar), and the same pre-defined scenario selection that has been around since the original game if "Normal" is too boring for you. Also since 2.0 players have been able to set the frequency of "Extreme Planets", the type that you have to research to colonize. You can't disable these planets entirely as the lowest setting is "Rare", but if you want to play with most of Dark Avatar and Twilight's improvements with the faster pace of the original GalCiv II then I suppose setting it to rare can help. You can also change the frequency of random events, or disable them entirely.
There's a new victory condition added to the mix, "Ascension". Essentially Ascension consists of finding "Ascension" crystals spread around the galaxy, and building a starbase around them to harness their power and holding them as a timer begins counting down every turn. When the timer reaches zero, your race ascends to Godhood. Sound familiar? That's because that's also the result of the technology victory, a bit redundant I think. Still, this "King of the Hill" style victory condition can be entertaining from a gameplay perspective even if the result is attainable via another method.
Modders can also have fun with this expansion as it comes with a pretty large mod suite. A Map editor for designing your own Galactic Maps of course, a Scenario editor so you can create a specific ruleset for a particular game, a Campaign editor so you can combine a few maps and scenarios into one story based campaign, a Tech editor that allows you to actually create new tech trees, or modify the existing ones, a Ship editor allowing you to import your own models for use as in game ship components, and finally an Improvement editor that allows you to craft your own planet improvements. I haven't spent a lot of time with these tools since I'm not a modder, but from what I saw they were definitely a nice addition.
The only other major additions are updated graphics adding more detail to ships, starbases, planets, and nicer looking planet screen art. The planetary invasion screen has also been revamped, and it looks a lot nicer in my opinion. There's a new type of Starbase added called the "Terror Star", a spherical mobile battle station capable of destroying an entire planets by wiping out the star system's sun. Sound familiar? It should, the developers even poke fun at this by having a special movie play upon completion of your first Terror Star saying "That's no moon...".
The real highlight of this expansion is actually the new tech tree system. Every major civilization now has its own tech tree, there are some technologies shared between civilizations of course (like the Universal Translator), but you're no longer playing the exact same faction with a different skin when you switch from the Terran Alliance to the Drengin Empire. If you play with a custom race you can also steal one of the major race's tech trees as well, along with their Dark Avatar Super Ability. I'm not sure what to think about this system personally, one of the nice thing about GalCiv II was that you could have a favorite race and not worry about balance, or using a specific race for a specific playstyle.
Twilight of the Arnor is definitely a must have purchase if you enjoyed the original game and the first expansion. It's also a must have if you like modding due to the surprisingly large amount of editors that come with it. I have mixed feelings about the new tech tree system, and I personally found the Terror Star to be rather useless (it can be destroyed by just one small fighter). Still, I would call Twilight of the Arnor the definitive version of Galactic Civilizations II.