For anyone who doesn't know, Malfi is a world that is prominently featured in several of the
40k RPG sourcebooks. It's a hive world that's the second most heavily populated in the Calixis Sector after the capital. Its main industries are engineering and metalworks, so your typical hive world factory stuff.
Malfi is infamous for how corrupt it is, and how many of its nobles have turned out to be insane megalomaniacs. The Radical's Handbook for Dark Heresy heavily implies that this is because some Recongregator Inquisitors tampered with the Malfi noble houses genetics, making them smarter and stronger willed, but as a side effect very prone to being total psychos.
Malfi has an insane amount of dangerous intrigue going on, partly because of all the nobles trying to assassinate each other, partially because the Malfians think they should be the capitol of the Calixis sector and many of them are actively trying to supplant Scintilla, and partly because of all the cults, particularly a Chaos cult called The Masqued which is rampant, particularly among the nobles.
In the Hammer of the Emperor book for the Only War
RPG (the guardsmen one) it mentioned that Malfi is one of the largest sources of recruits for soldiers to the Spineward Front, but they get no rules and no in depth background, which is kind of a shame because a Malfian regiment sounds very interesting to me.
Is there any more info out there on Guard regiments from Malfi?