Forgotten Hammer flick with Peter Cushing and Oliver Reed
This is an obscure Hammer production from 1962 originally called "Captain Clegg" based on the first book in Russell Thorndike's Doctor Syn series - "A Tale of Romney Marsh," which was published in 1915. The title "Night Creatures" is a misnomer while "Captain Clegg" doesn't work for reason I can't explain. The working title "The Curse of Captain Clegg" would've been more fitting.
In any case, this is an 18th century costume melodrama with skullduggery and quite a bit of action, plus some spooky bits. The presence of Oliver Reed and Yvonne Romain naturally bring to mind "The Curse of the Werewolf" (1961) and this is cut from the same cloth, just minus the lycanthrope trappings. "Witchfinder General" (1968), aka "The Conqueror Worm," is very comparable. "Black Death" (2010) was no doubt inspired by it, especially the mysterious marsh sequences.
Reed was in his prime and the stunning Yvonne Romain has a more significant role than "Curse of the Werewolf." Meanwhile Cushing was at the top of his game as the interesting protagonist.