What is the most interesting seed, thread, plot, or minor thing you've found mentioned in the books, or single book, in the line?

For example in DH2 (as I'm posting this in all the game threads), I found the Skydock 4/4 heresy. I found it pretty interesting and there's a few ties across a couple of the books as well, but it's by no means prominent in the line.

Basically looking for things to look into in the lines to have a good idea of one game I'd like to run from each core.

Grendel https://1d4chan.org/wiki/The_Guy_Who_Cried_Grendel have become fluff in dh1 book.

As someone who studied the Divine Comedy in my undergrad days, I find all the references to it in the Screaming Vortex to be intriguing. IRL 'Ghibbelines' and 'Guelphs' were two political factions in renaissance-era Italy. Following the Divine Comedy, imho, the central thrust of the Screaming Vortex's story arc should be questing deeper into it, to gain the ability to leave it and, in this case, conquer some of the Imperium.