Maybe I'm crazy, but I swear I remember somewhere in one of the books released so far that they gave details about the world of Bastion, the forbidden world. Our GM decided to use it as an off course destination once but had no idea about it. But I'm sure there were details about it in one of the books…or am I crazy? If I'm not, where in what book is it?

As far as I know, no book has actually published information about Bastion the system. There is the Bastion System Defense Craft in Battlefleet Koronus, which may be what you're thinking of.

Sorry, mate. The only "Bastion" entries I could find were these:

There is a Bastion class commerce vessel which is a filthy xenos vessel found on the Eastern Rim. (The filthy xenos in question are the Demiurg, which are definitely not the S*****. No, really.)

There is also a Bastion sector, which is the settings for the "Blood Gorgons" novels from the Black Library.

As for FFG's published stuff, no match.

Cheers,

- V.