Halo: Nightfall Second Stories
From Halopedia, the Halo wiki
Halo: Nightfall Second Stories were bonus clips originally unlocked through watching episodes of Halo: Nightfall on the Halo Channel. The clips were later put into collections to expand their target audience.
Second Story Collections
Classified ONI EYES files reveal the UNSC’s attempts to discern the methods and means by which a deadly new element was collected, trafficked, and ultimately employed against the colonists of Sedra. 
Nightfall: Human Element
Through classified ONI EYES files, this Second Story collection brings you face to face with those most impacted by the deadly attack on the outer colony of Sedra.
Go deep into the classified ONI EYES files that detail the sometimes harsh reality of ONI's constant involvement and meddling to find ever-elusive answers by any means necessary.
List by episode
The following list details what Second Stories were unlocked in what episode of Halo: Nightfall: