Most Powerful Monsters

It may be interesting to consider the most powerful monsters listed in the OD&D rules. Here's a list of monsters with potentially 10 or more Hit Dice:

1. Purple Worms (15 HD)
2. Giants (8-12)
3. Dragons (5-12)
4. Hydras (5-12)
5. Efreet (10)
6. Black Pudding (10)

One thing you can see is that for the time, the mainstays of Giants and Dragons are definitely near the top of heap of most powerful monsters in the game. (This is before power inflation required them to swell in hit dice in 2E and 3E games.)

The only thing more dangerous are the tremendous Purple Worms. Actually, I really like that flavor -- the most dangerous creatures in the world, blind in the underworld, burrowing incessantly "just beneath the surface of the land" (Vol. 2, p. 15; compare, for example, to the monstrous "Dholes" in various H.P. Lovecraft stories).

The other thing that's odd is the appearance of the Black Pudding monster, described as just "another member of the clean-up crew and nuisance monster" (Vol. 2, p. 19). Note the extraordinary strength of this creature, as shown by its very high Hit Dice (and brutal 3 dice of damage, the most in the game!) Perhaps the creature either needs to be as enormous as other creatures listed here, or be extremely rare due to some supernatural or unearthly part of its makeup. (See the picture in Supplement I: Greyhawk, p. 14. Other ooze-types like the ochre jelly, green slime, gray ooze, or yellow mold have only a fraction of the same hit dice. Note also that only the ochre jelly appears on the wandering monster tables.)

The list is modified somewhat if you take into account the special hit point accumulation for Dragons by maturity. In particular, a Very Old (6 hp/die) Gold dragon has 12 x 6 = 72 hit points, which is on average the same as 72/3.5 = 20 hit dice. In other words, it's the most powerful creature in the game (unless there's a great-grandaddy Purple Worm somewhere that you rolled all 5's and 6's for its hit points). The same can be said in this case for Hydras, of course.

In addition, there are notes in the text that indicate the possible existence of even more powerful creatures. Sea Monsters start as Purple Worms, and increase to 2 or 3 times that size (30 to 45 hit dice; Vol. 2, p. 15)! Rocs also can increase to 2 or 3 times the basic listing (up to 18 HD total; p. 17). Animals are considered up to a Tyrannosaurus at 20 HD (p. 20).

Finally, If you add Supplement I: Greyhawk, there are other rare and powerful monsters. These include Titans (effective 25 HD), Storm Giants (15 HD), Golems (11, 17, or 23 effective HD), Giant Slugs (12), Beholders (11 or so), and Liches (10+). Note that some of these creatures are actually proposed in the original set (Vol. 2, p. 21), along with ideas for super-strong Cyclopes, Juggernauts, Robots, etc.

Honorable mention goes to the following (8-9 Hit Dice): Vampires, Gorgons, Chimeras, Treants, and Invisible Stalkers (plus Will O'Wisps and Umber Hulks from Greyhawk).