(This idea first cropped up here, and was elaborated upon by kelvingreen and Milk.)
Old Johnny is a crayfish who has lived in the same lake for millenia, over the course of which he has grown slowly bigger and bigger; he is now roughly the size of a blue whale. The lake is a big and deep one - a Loch Ness or a Lake Champlain - and while Old Johnny used to live close to the shore he is now big and tough enough to roam the murky depths of his watery realm.
The people living around the lake worship Old Johnny as their god. They have worshipped him for so long, and done so in sufficient numbers, that he has actually become a god of sorts; an unthinking, voracious, alien god, but a god all the same. Old Johnny "knows only his own cold hunger" (Milk's line) and unconsciously wills his worshipers to assuage it. They do this by spreading rumours around the neighbouring lands that there are several giant crayfish, ripe for the eating, in their lake - and all are welcome to try to catch them for a small price in gold. Old Johnny then feasts on the unwitting adventurers, fishermen and thrill seekers who are attracted by these rumours.
Old Johnny: Neutral Gargantuan Crayfish
Movement: 6, Swim 12
Hit Dice: 16+6
No. of Attacks: 2
Damage/Attack: 4-24/4-24 (on a natural 19 or 20 on Old Johnny's 'to hit' roll, his claws have severed a limb; roll 1d4 to determine which)
The village itself is situated in an old, abandoned castle by the lakeside. There are around a hundred and fifty permanent inhabitants and worshipers of Old Johnny, forty or so of whom are combatants. They sometimes operate as bandits in the nearby hills, killing wayfarers or capturing them alive in order to feed them to their shellfish master.
Villager: Neutral Evil Human Bandit
AC: 8 (boiled leather)
Hit Dice: 1+1
No. of Attacks: 1
Damage/Attack: 1-6 (spear)
Their current leader is Chief Graham, a tall, thin man with icy green eyes and a pallid complexion. He dominates the rest of the community, calling himself the Voice of The Claws and bullying or cajoling or bribing the other villagers to do Old Johnny's will whenever their faith wavers.
Chief Graham: Lawful Evil Human fighter
AC: 6 (studded leather)
Hit Dice: 3+1
No. of Attacks: 1 or 2
Damage/Attack: 2-7 (spear) or 2-9/2-7 (broad sword and hand axe)
Chief Graham's wife, Lucinda, is the village wise-woman, shaman and priestess of Old Johnny - she guides worship and prayer to the crayfish and also functions as a kind of hedge witch and healer.
Lucinda: Lawful Evil Human ClericA picture of Old Johnny's Lake with the abandoned castle in the foreground:
Hit Dice: 2+1
No. of Attacks: 1
Damage/Attack: 1-4 (club)
Functions as a 3rd level cleric
Old Johnny's lake can be part of a mid- or low-level adventure. If the former, the players will face combat with Old Johnny and the villagers; if the latter, it is more likely that the players will be trying to escape with their lives.