Rebirth

Captain Kalliston of the Thousand Sons, taken captive in the shattered ruins of his homeworld, is tortured and interrogated by an unseen foe. He is sure that he knows the nature of his captor, and plays a dangerous game to get him to reveal himself. But the truth is very different and far more dangerous than he thinks, and he is in more danger than he could possibly imagine...

Read it because
It's a look at the shattered ruins of Prospero and how it feels to lose your Legion, your world and your identity. And speaking of identity, the guest star will make your jaw drop...