Some captain of the Grey Knights. He banished a Lord of Change named M'Kachen back to the warp for a century, causing the daemon to be butthurt and try to claim the captain's soul three times. Each time, it lost the battle and got kicked back into the shitpit. Stern also fought in the siege of Vraks alongside Inquisitor Lord Hector Rex, commanding the Grey Knights who banished the Khornate daemon prince Uraka the Warfiend.
Stern is one of two Grey Knights characters admired by /tg/ largely because he doesn't carve names into Daemon Primarch's hearts or other Mary Sue bullshit; no. Stern has character flaws like how he has been cursed into a constant battle with a particularly cunning Lord of Change, one which he knows that at some point, he will lose. If anything, he is doing his damndest to make sure that when M'Kachen finally gets the upper hand against him, he can at least attempt to foil his grand scheme and minimize the damage that it'll do to the galaxy. In other words, he is actually heroic in that he is willing to waste his entire life to deny one greater daemon the chance to unfurl his terrible plans that always involve him for its own petty reasons. He is essentially on an Ahab-esque quest, but at times he's the one being hunted.
He's also given us this quote:
"There is nothing in the arcane and blasphemous arsenal of the forces of Chaos that can compare to faith. With the power of faith, our weapons become shining instruments of deliverance that can cleave the mightiest daemon in twain. With the power of faith, our minds appear as slivers of pure agony to the daemon, driving into the wretched forms of those who would dare stand before us. With the power of faith, our words become commands that cause the daemon to cower and cringe in terror. I could meet my enemies unarmed without a shred of fear in my chest, for I know that the Emperor watches over me and guides my hand. So let them come. We shall show them what the power of faith can do."