The following text contains plot spoilers.
It is revealed that the Psychic Scourge draws upon Warp energy through Drogan's Power Source. As described in Drogan's audio logs, its function is to focus excessive amounts of warp energy into a chosen xenos species' specific warp signature, in theory causing massive injury and haemorrhaging to the creatures' central nervous system (or equivalent organs). In practice, however, he noted that it caused most target species' heads to violently explode, a side effect that was deemed an acceptable deviation from expected results. Successful tests were performed on Ork and Tau specimens, and very notably, a Tyranid Lictor, proving that the weapon would be an immensely valuable asset to the Imperium if perfected and deployed.
Their hand forced by Grimskull, Captain Titus decides to take Drogan's hesitant suggestion and fire the Scourge at the Orbital Spire to stop the Ork horde in time. To their surprise, the Orks seem to be fine, but a Warp rift opens and Bloodletters swarm through, followed by Nemeroth and other Chaos Space Marines. Nemeroth soon reveals that the real Drogan has been dead for a while, and the one the Ultramarines have been following is a "puppet" animated by his sorcerous powers.
A possible the reason that the Psychic Scourge discharge failed to kill any Orks is that the daemonically possessed "puppet-Drogan" did not set the weapon to target Orks, but instead deliberately tuned it to use the Power Source's prodigious power output to open the Warp Rift.
Inquisitor Drogan's demise, the destruction of the Power Source, the damage to the Orbital Spire and the Orks over-running Drogan's laboratory complex likely mean that the design of the Psychic Scourge is a secret that is now forever lost to the Imperium.