They were called Fire Hawks and they disappeard in a warp. They've got mutated by it - the mutation caused their bodies to rot but they still kept the faith to the Big E. With only about 200 brothers being "alive", they painted their armors black with death decorations and they also abolished all the ranks. Now they pop out of the seemingly nowhere, saving loayalists from the worst and disappearing again. Technically they are space marine spirits/apparitions with their armor covered in warpfire and some witnesses say they saw decayed bodies or bones through the cracks of their damaged armors.