Inquisitor Thrax is a member of the Imperial Inquisition, presumably of the Ordo Hereticus or Ordo Malleus, given the nature of the incident he is responding to.
The following text contains plot spoilers.
Thrax appears in Chapter 18: Epilogue, arriving on Graia with an escort of Black Templars to investigate Captain Titus for signs of heresy, due to Leandros' suspicion that he has fallen to Chaos. Titus agrees to cooperate after Thrax states that his entire Company and everyone associated with him will be assumed to be corrupted if he resists, and dealt with appropriately. Titus is then taken into Inquisitorial custody. However, contrary to standard procedure, Thrax does not ask Titus to disarm himself, strongly implying he does not actually believe Leandros.
- Thrax is also the name of an Inquisitor who becomes corrupted and possessed by a Warp daemon in a short piece of background fiction found in an old sourcebook (Realm of Chaos: Slaves to Darkness, Games Workshop 1988).