Many Owls |
Fortress of the Nail is a Society Scenario in a Hellknight fortress, so if you need Hellkninghts, well, that's where you'll find them! Truth be told, there isn't a lot of combat against them (if that's what you want), but still, lots of interaction. Haven't played it, so no idea if it's actually good or not.
Latrans |
The 2nd and 3rd parts of the Fiend trilogy: 2-10 Fury of the Fiend, 5-11 Fate of the Fiend. (0-22 Fingerprints of the Fiend have no Hellknight but gives more context to the site.)
Hellknight play a big part in is 7-25 Orders from the Gate. the Hellknight are allies and can allow free retraining to the Hellknight Prestige class.
There are a couple others, but they usually are very minor.