quietben -
It is as DoomBringer90 said. That is, Gal Civ2 originally came out as DreadLords (DL). Then, a revised version was issued, Dark Avatar (DA). Lastly, the Twilight of the Arnor (TA) version came out.
Each of the three versions - DL, DA, and TA, for short - have different features, mainly adding features and complexity with each successive version. I have played DL and DA, bit not TA, so I was not prepared to rule out that some sort of "demand surrender" feature had been added to TA.
The other games (SEIV and MOO2) are/were turn-based, space expansion 4X games like GalCiv2 but had several differences, one of which was that both could be played multiplayer. They had many similarities, such as the ability to design ships based on research choices, and differences, such as tactical control of space battles. Google is your friend here, just search on MOO2 and/or SEIV. MOO2 came out around 1996 and SEIV around 2002, IIRC.