Shades of Dark is book five in the Dark Universe setting by Jason Halstead

For Aden and Twyf, a human and his Tassarian lover, the time has come for them to have a few moments to themselves. The moment is spoiled by  a surprise attack launched by the Kesari. Their goal? Destroy anyone and everyone seeking to upset the control they've established over the universe.

The thing about heroes is that, no matter how unlikely or unwilling, they don't go down without a fight. Bloodied by the assault and the loss of their friends, Aden and his remaining friends must set out to do the impossible: Topple an alien regime.

Hopelessly outnumbered and facing superior technology, Aden, Twyf, Sierra, and Seph have run out of tricks and luck. Will running on sheer willpower alone in an alien universe amidst uncertain allies be enough?

Look for these other Dark Universe stories:
Book 1: Into the Dark
Book 2: Out of the Dark
Book 3: Chasing the Dark
Book 4: Dark Star
Book 5: Shades of Dark