Asmodai
Oh shit......
ASMODEUS POISONTEETH Asmodai is the Master Interrogator-Chaplain of the Dark Angels. He is the oldest and most experienced of the Interrogator-Chaplains (hence his rank), and also the most obsessed with hunting down the Fallen Angels and making them repent. SHOWING HIS INTENSE BURNING LOYALTY TO THE GOD EMPEROR.
He is NOT the Grand Master of chapter's Chaplains however, that position belongs to Sapphon, but he's not as famous as Asmodai.
Biography
Starting as a no-name tactical marine in the eighth company (that's what the book says, but the 8th company are Assault marines) Asmodai was a fairly unremarkable soldier. It was only when he was the lone-survivor of his first encounter with one of the Fallen Angels when his career actually took off: as there was no-one else to explain the situation to, Asmodai was taught the secrets of the chapter and was offered a position in the Ravenwing or the Deathwing. He chose neither and declared he would be a Chaplain instead as his obsession with finding the Fallen and exacting revenge started taking root.
He captured his first Fallen in 411.M41, but failed to make him repent when the Fallen died without confessing. This obviously caused Asmodai to enter deeper levels of RAGE and then started going to unhealthy lengths to capture more to slake his angry streak. With renewed vigour, Asmodai caught three more Fallen over the next 25 years, but still managed to kill each of them before they could repent. So the Inner Circle decided to give him one they'd caught earlier before Asmodai exploded with pent-up frustrations. Asmodai broke this one in one single eighteen day session of physical and mental torture. When Asmodai emerged from that cell smoking a cigarette, he had earned his first black pearl and he smiled for the first time.
His obsession has grown and bled into the other aspects of his life as a Space Marine, and he's basically become the "fun police" of the 41st millennium. When Asmodai bombarded an Imperial city to flush out a single Fallen, Azrael had his command privileges revoked and transferred him to the 10th company so he could learn humility. However, when a bunch of recruits didn't answer his questions as enthusiastically as he liked, he ordered them all killed because he thought their gene-seed might be contaminated by "tardiness". When he heard someone from the Seventh Company laughing while inside the Rock, he had the entire Company sworn to silence except for hymns and communicating in battle. He doesn't actually have the authority to do any of those things, though. At least, not without Azrael's approval (which he won't get), he just tends to threaten the officer in charge when he doesn't get his way. Besides, a vow of silence covers speech. Laughing is not speaking.
Unfortunately this zeal to track down Fallen has led his chapter into some big troubles, like where he countermanded a Captain's orders and diverted a Dark Angels strike force from their position fighting Orks to investigate rumours of a "remarkable warrior". It turned out that warrior was in fact an Inquisitor of the Ordos Xenos, and that the Dark Angels unauthorised repositioning had cost the allied Silver Skulls most of their army. Not only that, but the Inquisitor returned to the Silver Skulls homeworld to explain exactly what happened and demand assistance in punishing the Dark Angels for their behaviour. Wisely (and partly for fear of going head-to-head with the Unforgiven) the Silver Skulls Chapter Master instead requested that Azrael come and explain himself personally. So Azrael had to set aside his duties for a time to go and apologise for what happened. During that time Asmodai was grounded under lock and key until the Supreme Grand Master got home to decide what to do with him. When Azrael returned to the Rock and asked if Asmodai was sorry yet, Asmodai responded that he was sorry he didn't kill the Inquisitor when he had the chance.
Azrael has had to rein him in or cover up for some of his more excessive actions, but he keeps Asmodai around because, say what you will about his sense of humour (or lack of it), he delivers -- if there's a Fallen Angel (or even someone who might know something about a Fallen Angel) out there, he will track it down. Interesting that despite his long service and obsession he has only able to make two fallen repent and apparently killed more by accident, implies he's really just a sadist asshole. His preferred tool of interrogation is a torture weapon called "the Blades of Reason".
Quite ironically, Asmodai is named after a daemon king of lust in Christian mythology. So while WH40K Asmodai is busy fun-policing the galaxy, daemon Asmodai is conversely busy living it up, Slaaneshi-style.
Tabletop
Asmodai made a semi-welcome return to the tabletops in the 6th Edition codex with new rules and a new model. Unfortunately his effectiveness is fairly lackluster.
He is a standard interrogator chaplain with no upgrades, except for the Fear USR and his trademark "Blades of Reason", which is a no-frills close combat weapon with the Instant Death rule... he didn't even get a bolt pistol until GW FAQ'd him to have one.
Fear doesn't help him so much, since his WS is high enough that he hits most of those things susceptible to Fear on 3+ anyway and dropping those things from 4+ odds to 5+ odds when they hit back is not something to be jumping for joy over. Neither is an instant death weapon that has no bonuses to-wound nor any AP value; if you get REALLY lucky you might insta-gib a monstrous creature by rolling a 6 to-wound and it failing its save, but you're still probably better with the more reliable Crozius against those anyway, while you were always insta-gibbing humans/tau/eldar with that crozius to begin with...
Unfortunately, you are probably much better served taking a tooled up nameless Interrogator Chaplain in your force instead of Asmodai, which is a shame since his fluff is so hilarious.