It's Hallowe'en, so I really ought to talk about monsters. I'd like to share my thoughts on the king of monsters -- well, kings of monsters, since it's not a single uniform type -- in the original books. No other monster in Volume II, Monsters & Treasure, is quite as dangerous as this one. Sure, others may have fancy powers, like the troll's regeneration or the vampire's many powers. But this one has the most hit dice, and thus the best combat ability. And two special attacks that will kill you dead very quickly, if you aren't careful.
And there's something in the description in the book that most of us miss. They are everywhere just below the surface. Everywhere. I know I don't use them enough, myself, and I bet you don't, either. Certainly not enough to qualify as "everywhere". We're too worried about pitting low-level PCs against such a tremendous force.
No, it's not the dragon. You probably thought "dragon" first, because as the old cliché goes, it's "Dungeons & Dragons", so the dragon is the most fearsome embodiment of what adventurers rally against.
But it's not the dragon.
It's the purple worm and its relative, the sea monster.
Look at the purple worm first. It has 15 hit dice, more than the most powerful dragon in M&T. It doesn't have armor that's as good as a dragon, and it's slower and can't fly. It has a deadly poisonous stinger instead of a visually-spectacular breath weapon. But it's a pure embodiment of hungry hate. Dragons? They sleep too much, and spend their waking moments gloating over their treasure. Purple worms can have treasure, but they don't care about it. In fact, the book doesn't say, but the treasure is probably inside the worm. Good luck getting that with mere trickery.
Why would it be inside the worm? Because its most powerful attack isn't its instant-death sting. It swallows ogres whole. Gotten yourself swallowed? You're dead in 6 turns (presumably "combat turns".) You're gone, no Raise Dead possible, in 12 turns.
And the sea monster is the aquatic equivalent of the purple worm. M&T tells us that the small sea monsters have stats identical to the purple worm. Larger ones have double or triple hit dice. Up to 45 hit dice! I'm guessing things that size can swallow dragons as well as ogres.
A lot of campaigns go with dragons as their quintessential monster, the apex predator of D&D. These dragons usually get beefed up, made more intelligent and given powerful fear auras, turning them into diabolical masterminds. Others go with an undead theme and place the lich or a super-vampire at the top, again as a mastermind. A few go with demons and devils, or a Lovecraftian equivalent, again as masterminds. But few people remember that Lovecraft's most powerful entities were mindless and chaotic, the embodiment of entropy and decay, or that he included a vision of the final fate of Earth: consumed by dholes, the giant, mindless (and possibly purple) worms.