Enormous Spider

"This is the best room in my inn"
illustration by Gustave Dore in La Legende de Croquemitaine by Ernest L'Epine (1870)

In October I wrote up several monster entries based on "lost" monsters found in the module B2, including Warrior Zombie, Giant Rat King, and Phantom Cat. Other "lost" monsters for Holmes Basic can be found right in the Sample Dungeon by Dr. Holmes himself, including a giant snake, giant octopus and an ape, none of which appear in the Monster List in the Blue Book.

One that is particularly obscure is the original version of the spider in Room J. This spider lurking under the ruins of the tower of the wizard Zenopus was possibly inspired by the Conan tale The Tower of the Elephant (Robert E. Howard, 1933). In that story a giant spider drops on Conan from the ceiling of a room in the tower of a sorcerer.

If you have the 2nd or 3rd edition of the Blue Book, the spider in Room J is named a "giant spider" and essentially matches the entry in the Monster List at page 32: HD 4+4 (21 hp)  with a poisonous bite (2-8 points damage). However, it also has an AC 3, which is off by one from the standard AC 4. This is a residual reference to the original version found in the first edition of the Blue Book, which was more ferocious: in addition to the lower AC, it has HD 6 (31 hp) with a poisonous bite (1-8 points damage) that saves at -1 "because it is so strong". Furthermore, the spider was originally referred to as an "enormous spider" rather than a "giant spider". This reflects that the first print of the Blue Book (~mid-'77) did not have a Monster List entry for spider. The entry for "Spider" (including huge, large and giant types) was added to the 2nd edition (Nov '78), and was adapted from the Monster Manual entry (published in Dec '77). OD&D doesn't have a codified entry for Spiders of unusual size. There are Giant Spiders mentioned in the variable damage and wandering monster tables in Greyhawk, but no full entry apart from "Large Insects or Animals" in Monsters & Treasure, which can have HD  2-20, AC 2-8, and damage 2-4 dice. Holmes may have simply picked a set of stats from these loose OD&D guidelines to generate his enormous spider.

Here's an adaptation of Holmes' original spider foe for Holmes Basic or OD&D:

Enormous Spider

Move: 20 feet/turn; 100 in web

Hit Dice: 6

Armor Class: 3

Treasure Type: D

Alignment: neutral

Attacks: 1

Damage: 1-8

Wicked old giant spiders that have grown to enormous proportions. Their bite is occasionally less damaging than that of a giant spider, but their poison is worse (-1 on the saving throw dice). Less swift than younger spiders, they favor a silent sneak attack. The spider, concealed in a darkened ceiling area, drops onto an unsuspecting victim. A successful hit roll indicates the victim is knocked down and surprised; the next round the spider has first attack while the victim fights at -2 as they stand up. A missed hit roll indicates the spider lands on ground next to the victim and the fight proceeds normally.