"The Were-Snake" by Frank Belknap Long

Published in Weird Tales in Sep, 1925

Messalina: the third wife of Claudius, and cousin of Nero. She had a reputation for promiscuity and for cruelty - seeing to the execution of several of her husband's family members, especially the women. In the end, she conspired with Gaius Silius to overthrow Claudius, but was ratted out by Narcissus and executed.

Lord Clayton-Maddox: I believe this is a fictional person. Which would be expected, you don't use the real names of real and powerful people in a work of fiction unless it is extremely flattering. And even then probably not.

John Dee: 16th century English mathematician, astrologer, occultist, alchemist, antiquarian, and Hermetic philosopher. He was the court astronomer for Elizabeth I.

3 yards: for you metric types, just call it an even 3 meters.

Acheron: a river in Greece said, in Greek mythology, to the entrance to the underworld of Hades

Caliban: son of the witch Sycorax, from William Shakespeare's play "The Tempest"

The picture used is the illustration for this story from Weird Tales by Andrew Brosnatch.