Soul Drinker - Following a spectacular break from the Imperium, Sarpedon of the Soul Drinkers has just learned that his Chapter's rebellion, their mutations, the destruction of Ve'Meth - Daemon Prince of Nurgle - were orchestrated by a Daemon Prince of Tzeentch named Abraxes, who is offering the chance to abandon the Emperor and join the servants of Chaos. Sarpedon informs him, "This Chapter belongs to no-one," and then rams the Soulspear, the artifact at the heart of all this, straight into the Daemon's heart.
Sarpedon gets one of these at the end of at least the first three books. In Bleeding Chalice he manages to not only convince an Inquisitor that he's still on the side of the angels, but also takes out the god-mutant Teturact, who has been building his own empire. In the third book, he disables Tellos for a decent period of time (read: long enough for the Inquisition to take out the entire planet) by literally dropping a bridge on him.
Sarpedon may be a walking CMOA in Phalanx but he isn't the only one:
Librarian Tyrendian: Tricking a Great Unclean One into eating him so that Luko can escape, Tyrendian gathers up enough psychic lighting to destroy the Greater Daemon, causing the Daemon Army to panic and fall back.
Sister Aescarion: Undergoing a horrific Mind Rape by Abraxes while trying to close the Predator's Eye warp portal, seeks something that can resist Abraxes. She finally finds it when she remembers her faith in the God Emperor, knowing that there is nothing Abraxes can do to shatter it. Abraxes goes absolutely nuts when he realizes that he cannot break her faith.
Captain Luko: Fighting through a veritable hell to reach Abraxes, whose continued existence Luko considers an insult to everything the Soul Drinkers have achieved since they rejected him, manages to blind side him while he's performing the aforementioned Mind Rape and hold him place long enough to have the Warp portal sever him in half. Then, when the battle is over, he helps carry a crippled Sergeant Graevus into the portal, showing that the Soul Drinkers die free.
The very first action the Soul Drinkers take after the Imperium turns on them is a massive one. After the Administratum refuses to have the Mechanicum return the Soulspear and for the Soul Drinkers to leave the Star Fort they were tricked into taking for them, they deploy a massive orbital bombardment cannon to drive out the space marines. Learning of this, the Soul Drinkers take a flight of fighters so old they're effectively museum pieces, fly at the cannon, hijack it, and turn it on the Imperial fleet. Better yet, when they fire on fleet they managed to only disable them with minimal casualties rather than outright destroying them - and just to flip them off blow up the Star Fort as they escape.