I'm not so sure about cthonian stars. I'm not sure cthulhu and traveller are going to make a good mix. I mean, some things just don't mix with cthulhu.
As for a horror themed traveller game, well, now, let me think a minute....
How about the concept that grandfather intends to return to this universe and essentially enslave the whole damn galaxy? Maybe grandfather has a neat idea involving engineering the whole galaxy, a'la perhaps what the AGRA intelligence was doing in 2300, and really doesn't give a frak what happens to the lifeforms here already?
The idea of grandfather launching an all out conquest of the known galaxy with an unlimited array of weapons, biological, nanotech, stellar scale, etc, ought to provide all the horror any traveller fan could ask for, and then some.