The Tyrant Star, the Hereticus Tenebrae, the Widening Gyre, Komus... the central mystery of the Calixis Sector, and we never got a **** answer about what it is. With the shift to the Askellon Sector, we may never know.

Is there any official fluff I'm missing (i.e. anything other than the texts in Disciples of the Dark Gods)? Barring that, does anyone have any good fan theories?

There's a FULL post about it in the GM section which is a GREAT read, bursting with ideas and connections to the Haarlock legacy and cues that people have uncovered over the years!

Check it out!

I always assumed that it was mostly left blank to allow you to make it whatever sort of final boss or central problem you wanted it to be. Thanks to a discussion with a player while planning Black Crusade, we made it the birthing chamber of a fifth Chaos God, created by the lies and contradictions of Calixis (We liked the theory that Drusus' rebirth was a lie and that he was a daemonhost, a case where Radicals actually, really succeeded in using Chaos on Chaos on a sector-saving scale and then built a massive conspiracy to hide it and promote a false saint). Unfortunately for the sector, as this discussion was done before a Black Crusade game...well, I've posted elsewhere about what ended up happening, but suffice to say Calixis is very much on fire right now in my 40k RPGs and paying deeply for all its lies.

Has anyone else actually had the Scarystar come up in their games? And if so, what did you end up making it, exactly, as the original poster asked?

As others have said, I think the lack of detail was deliberate. I doubt Black Industries even ever intended to finally reveal what the Tyrant Star was, in order to 1) leave it up to the GM of any given campaign and 2) because so often when you solidify these kind of ideas, they are never as awesome as people might have thought. I certainly felt this with the Deathwatch mystery which turned out to be "Lol, Necrons are hiding inside stars!" That is unless the Haarlock Legacy trilogy is meant to solve it somehow, but then the ending for that was a bit lame (see point 2) and didn't explain anything anyway, and so I hope not.

Myself, I don't even think players were ever meant to solve the riddle of the Tyrant Star. It is probably mostly intended to be a background thing, which players maybe encountered, such as if it made an appearance at a planet and the usual result of a the planet descending into chaos resulted, or be obviously connected to a secret, but not necessarily what the players are directly looking into (see the Haarlock legacy again). An unknowable, unstoppable force to give flavour to the setting.

I agree with Borithan at 100%, though I've hinted at what it could be, I've also given conflicting hints at what it could be

That might sound like a lazy answer, but ultimately it could all the options presented in the Core-book [in the Conclave section, as theories of various Inquisitors, or none of them.]