MY FLAME BURNS FOR THEE!
Space Marines that have flaming tattoos, replace their teeth with flints, put flamers on Land Raiders (having invented the Land Raider Redeemer by replacing the Crusader's Hurricane Bolters with Flamestorm Cannons) and breathe fire.
We'll say that again. They, quite literally, breathe fire. Not thanks to any genetic mutation, but through being badass enough to swill promethium and then use sparks from their flint teeth to set their breath alight. Think the fire breathers found at rennaisance fairs, but with napalm instead of alcohol.
Possibly Matt Ward's second favourite chapter other than the Ultramarines, since he's put them into several of the Codices that he's written, including Space Marines 5th Edition, Grey Knights, and Necrons.). This makes them slightly less badass, although the least awesome thing that they've done thus far is taking credit for the work of the Grey Knights.
Their founding chapter is unknown; in the book Legion of the Damned it's stated to be the Imperial Fists, but both Deathwatch and the sixth edition Codex: Space Marines describe them as being of unknown heritage (and that it's suspected to be the Salamanders). One possibility is that they're Salamanders successors, but they're so awesome that the Imperial Fists let them into the Feast of Blades anyway.