DW Monsters: Sea Devils

Next up in season 9, we’re back to present-day Earth and The Sea Devils. The eponymous race are introduced as a variant form of the Silurians adapted to underwater life and, beyond the aquatic theme, provide a similar role in the TV show – although they do appear to be more inherently violent, presumably due to a warrior upbringing. They return much later in the classic run alongside the Silurians in Warriors of the Deep but were not seen again on screen until almost the end of the Thirteenth Doctor’s run. 

Outside of the TV series, they have proved less popular than their Silurian kin, appearing alongside the latter in the 1993 novel Blood Heat, but rarely featuring on their own outside of a few comics and short stories. They have appeared in just a couple of the audios, neither of which feature the Doctor and one of which is part of a larger Silurian plot arc. A mention in a novel that doesn’t feature them directly implies that they are the same as the Deep Ones created by H.P. Lovecraft - although beyond living underwater and being cold-blooded, there doesn’t seem to be much resemblance between the two.

D&D Monsters: Noncorporeal Undead

The concept of ghosts or other restless spirits is an ancient one, found, in some form or another in pretty much every human culture. The details vary considerably, and ghosts may not necessarily be malevolent, or even frightening but the basic idea is essentially universal. They're the sort of thing that just had to be included in D&D, albeit with the addition of generally making them more dangerous - and thus a meaningful threat - than they usually are in legend. There are several different types in the game, arguably a different sort of being from the corporeal undead, united by lacking a physical body and with various other means of inflicting harm. Four of these types are consistently present in the core rulebooks, although their details can vary significantly.