The Daughters of the Blackest Night focuses on powers and feats appropriate for infernal warlocks that form a pact with Glasya. The story content is short and somewhat entertaining. The sidebars are also of interest in that they mention optional restrictions that you could apply to the spells and feats to make players roleplay/work a bit harder for, in addition to a sample description reskin for Warlock's Curse and Shadow Walk.
I like the idea of warlock spells having variable descriptions depending on your pact, and even the sort of entity you hang with. Now only if anyone in my group would play a warlock... T.T
The spells themselves have a lot of strange quirks built in. For example, a lot of the daily attacks let you take damage to recall them, with a bonus on the next attack. My favorite however is hellsworn blessing, which lets you tag an ally to transform them into a devil of some sort in addition to granting them some hefty bonuses.
The feats let you do a lot of cool shit. While almost all require the infernal pact, they arent all particularly thematic to the Daughters (really, only three are). Allegiance to the Daughter causes every cursed creature you kill to erupt in a column of hellfire that damages things passing through, while Sickening Shadows inflicts automatic necrotic damage on any creature that misses you with melee attack while Shadow Walk is up.
Great article for any infernal warlock.