The final fate of Sir Amery and his nephew as the Chthonians manage to track them down across multiple continents before rooting them in place psychically then dragging them down into the Earth. Sir Amery's fate is also A Fate Worse Than Death.
Indeed, Sir Amery is a horrifying as well as they take his brain and put it in an artificial body made of Cthulhuoid substances before being tortured for information until everything he knows is given to the Chthonians. His hideous stinking gelatinous body is then used as their messenger.
The entire oil drilling story as the Sea Maid's crew find increasingly off things like seismic activity happening every five seconds, weird behavior of fish (weird, as in eating a hapless diver alive), and odd weather patterns. Eventually, a gusher is hit, only to be revealed to be of blood rather than oil. They'd drilled into the body of a massive monster which was less than happy with their action. It's a genuinely bit of Lovecraftian horror in a series which is more Lovecraft Lite.