Tempered by Pain: The Li-Tech Chronicles: Book 2

Mankind ruled through the liberal deployment of the Advanced Human Forces for thousands of years. Despite centuries of war, these elite soldiers never broke against hordes of enemy armies. With Light as their ultimate weapon, their empire expanded and settled fifty worlds while using Earth as a soldier farm.
With the fall of Earth, defense of the empire rests on the shoulders of anyone who remains. Recent graduates of the Algol Basic Training Program must take up their rifles to defend mankind and its resources from the Raak'Shee, XanRai'd, and others.
Jack and members of Turaspeir Squad must face the realities of war as darkness spreads across the universe, extinguishing the stars one by one. Their unknown enemy lies in wait, comfortable in the knowledge that Li-Tech is powerless against them. Unless Jack can find the enemy's weakness and break their immunity to Light, humanity is doomed to be absorbed by shadows.