Apologies for the bad quality photo, I swear I'll buy a new phone soon. My lens makes the picture all grainy and attributes to the chunky-looking paint.
I haven't got a lot of lore written out for them yet, but here's the main idea:
They are a Dark Angels successor chapter, and while they retain their predecessor's monastic rites, they are very separate from the bulk of the other Dark Angel chapters, as they are currently on a "penance crusade". They have a strong "knight" aspect to them, with a strict hierarchy and beliefs. The bulk of their chapter consists of Primaris Marines, due to the Cystan Heresy and the ensuing acts of terror.
The Cystan Heresy: The chaos marine army of the Death Guard assaulted the small planet of Cystan, hoping to gain a foothold near to the Great Rift. The PDF (planetary defence force) that had been tasked with defending the population fell swiftly, and the Nephilim intercepted a distress call that had been sent from Cystan in desperation. They sent a small detachment of their Chapter to scout out the planet and gauge the level of aid it required.
The scouting force soon lost contact with the orbiting Nephilim Vessel, the Angel of Alnar. This was due to both vox-warp interference and the awaiting hordes of Plague Marines, who had expected something like this to occur. The force was exterminated by their opponents, and had only just time so send a vox-hail announcing their status. The Angel of Alnar, scrambling to bring retribution upon the traitor Legion, bombarded the planet from orbit with a massive cascade of Drop Pods, almost all of which managed to make planetfall.