Brighthammer 40,000 is a mirror universe of Warhammer 40,000, as you can probably tell by the name. The sun rises on a galaxy filled with wonders and adventure, all held in check by the strict but fair gods of order, and everyone gets a chance to see the Emperor's rockin' goatee. Except the Tau, who are left exactly the same and thus look like nasty dictators.
If you are looking for the lulz, check out our original BrightHammer40k (1st edition)! This one has more of the air of "find-and-replace grimdark:noblebright" where even the Tyranids are strong yet benevolent, optimistic pioneers.
If you're in the mood for something less diabetic, proceed onward to Brighthammer 40,000 (2nd edition), citizen! This one leans more toward pulpy sci-fi, and we had to make our own lore instead of just inverting the Warhammer stuff.