What do we know of the oft-mentioned Inquisitor Herrod?

He is the one in the piece of fluff at the beginning of the Disciples of the Dark Gods book who descends into the bowels of the Bastion Serpentis for lore about the Tyrant Star.

He is often referred to with cryptic codes.

And (SPOILER BELOW) …

….

….

…. he makes an appearance in Dead Stars, when he is cybernetically resurrected as a metal monster after some terrible ordeals to continue his hunt for the Haarlock Legacy. But what happened to him, really?

The Laughing God said:

That's quotes, not codes …

I thought of him as finding esactly what he wanted in the Bastion Serpentis - and being driven irreversibly mad by it. Others in the Tyrantine Cabal cybernetically resurrected him (as more of a servitor than a person) in order to use him as a tool. It could be rather useful to have an automaton that is able to wield Inquisitorial Authority on demand in order to send him to places like Mara…

I remember reading somewhere that he had a run-in with the Vile Savants. DotDG maybe? Or Radical's Handbook?

Indeed Herrod pursued the Vile Savants, over seventy years ago, and it was the last time anyone heard from him, according to the text box on page. 188 in IH. Lots of stuff about Inquisitor Herrod there.

He appears in the third part of the Haarlock's Legacy trilogy of adventures, Dead Stars. The passage of time has not been kind to the man.

He also knows Marr from earlier time, probably served under/with him. IIRC DoDG, letter to Marr concearning investigation of some Rogue Trader.