midnight assassins
A reference to The Italian, or the Confessional of the Black Penitents, by Anne Radcliffe, published 1797. In particular, this is the title chosen for a popular pirated chapbook version of the novel, popular because of its cheap production and therefore low price (British Library).
a bilious fever
When referring to disease, bilious is an overproduction of bile. Drawn from Hippocrates's now obsolete Theory of Four Humors, bile was also connected to a bitter and choleric personality (U.S. National Library of Medicine, The World of Shakespeare's Humors).
To poultice chestnut mare” — a farrier’s bill!
A remedy for cuts, swelling, and infection, specifically common for treatment of hoof abscesses. The herb poultice would be applied by the farrier (horseshoer), who in regency times often doubled as a general veterinarian and animal dentist (BookDoors).
poison nor sleeping potions
Allusion to The Castle of Otranto by Sir Henry Walpole (1764). Complicated by the fact that the "poison" and the sleeping potion were one and the same, and the sleeping potion was only used to give the the appearance of death (Montague Summers, The Gothic Quest- A History of the Gothic Novel).
massy
"Consisting of a large mass or masses of heavy material; of great size and weight; massive. Of a building: consisting of great blocks of masonry" (OED).