What exactly is a terror star?

Evidently (I don't really know either) it was something like a Death Star. It could destroy planets or maybe whole stars. There's some talk of including that in an expansion pack.

Some of the xml files mention it, so it was nearly included in the game when it was released.

[UPIssue InternalName="TerrorStar01"]
[Question]As technology reaches levels never before dreamed possible, the cruel races of the universe will find ways to use this power in destructive ways. The Terror Star is one such example of this. A weapon powerful enough to implode a planet, it is a symbol of all that is wrong in the universe. We vote now on a 5 year restriction on Terror Stars, where no new production will be tolerated. How do you vote - should Terror Stars be banned in our galaxy?[/Question]

[UPIssue InternalName="TerrorStar02"]
[Question]Terror Stars promote mass murder on an intergalactic scale. As long as these destructive machines are present in the galaxy, universal peace will never be realized. For this reason, we'll be voting on a permanent ban on these vessels. Should the Terror Star be forever restricted under the laws of the United Planets Council?[/Question]

It destroyed whole stars. Since planets were tucked inside stars, this meant it destroyed a lot of planets.

It was going to be in the release, but they couldn't hook the AI up to them. It never got hooked up in galciv1, which meant the AI couldn't use or recognise them. The current problems (as I understand them) are NOT telling the AI to use them or calculate the threat they posses (we've seen the AI do things similar to this already, building it's own ships and countering yours), but teaching the AI to defend them.

Oh, and I think they decided this time it'll just kill planets, but I remember Cari saying Brad wanted the star to explode. So don't quite know which way it's going.