If you're still accepting alt-faction Cain ideas (I couldn't find the original prompt), I have one (that, for the record, I came up with before reading Darth Cain).
Ciaphas Cain: Champion of Khorne
It all started with a sore throat during an uprising.
Nothing too serious, nothing to impair fighting ability, or avoid the front lines and getting knocked out by a lucky blow from one of the cultists, before being (presumable) mistaken for one of them during clean up.
Only to awaken, get shoved out into the cultists ranks between Imperial battlelines and a group of Berserkers.
In such a situation, anybody, even a commissar, would be forgiven for fleeing the followers of Khorne in a dead run, screaming in fear.
Except, that sore throat mentioned earlier? It tends to deepen the voice somewhat, turning screams of terror into what could be mistaken as a warcry.
There was a moment before the sound of the Berserkers charge started, the lunatics not wanting to lose the glory to one of their cultists.
The Imperial lines, clearly in need of a Commissar's attention, broke.
I kept running, trying to reach Imperial lines before the Berserkers could catch up, but exhaustion overtook me first.
And upon awakening, a repeat on a different field of battle, then another and another.
Extract from report from Inquisitor [Redacted]
... and one notable member of the battlegroup [Redacted] is an unknown cultist first encountered in the [Redacted] Sector (where newly commissioned Commissar Ciaphas Cain disappeared).
While most would focus on the marines of the battlegroup, or even just those with highest casualty rates, this cultist with zero recorded casualties is nevertheless noted as the most highly favoured of [Redacted] in the battlegroup, going by his ability to break the morale of the Imperial Guard more effectively than the rest of the battlegroup put together.
When battlegroup [Redacted] deploys, this cultist will lead the charge, screaming a warcry. In all engagements thus far, the Guard has broken before he could reach their lines, which does not prevent him pursuing until he finally passes out.
As such, I must recommend deploying Space Marines, noted for their lack of fear for the cultists to take advantage of, to combat battlegroup [Redacted].
Except from response to prior report
There are greater threats to the Imperium than a single battlegroup, which demand the Marines attention first. And stop paying your censors by the word!