16th in the Divinities and Cults: Volume III series, we have Dagon, Mesopotamian demon lord of sea people and invasion.
1. The tide comes in. Ocean waters flood the immediate area, rising for 1d3 rounds at a rate of 1d3 feet per round (double if near the sea), likely making those caught within salty at best.
2. A giant crab skitters forth, being 3 HD larger if within range of the ocean! It will ignore any followers of Dagon and happily attempt to crush all others.
3. Arrrr! Like pirates, 4d4 sea raiders have descended upon the area, terrorizing the local populace, likely given to much drinking, plunder, and pillage. Will the party stop or help them? The local lords will certainly be in favor of the former.
4. Ahhh! Like locusts, 4d8 fish men have descended upon the area, either planning (if their numbers are less than), being in the process of replacing (if their numbers are about the same), or finishing off (if their numbers are greater than) the local populace. In any case, any locals who remain will know that something very fishy is going on.
5. The area within 60’ becomes overly moist and stinking, causing all to save vs. paralysis or slip and fall. Even followers of Dagon aren’t immune to this effect, though they’ll savor this manifestation of their fell lord regardless.
6. Suddenly, a large fish leaps out and attacks the party! It is a (roll 1d6):  giant catfish,  giant piranha,  bull shark,  great white shark,  giant rockfish,  giant sturgeon. If encountered on land, halve its HD and it will only last 2d4 rounds without water. If encountered deep in the ocean or in the presence of a cleric of Dagon, increase its numbers by 1d3 instead.
7. Dagon comes to you! All water in a 1/4 mile area becomes vile and dead fish-ridden, preventing its consumption and enjoyment by all normal folk. Followers of Dagon aren’t normal folk of course and actually find it to be quite delicious and pleasant.
8. Worshippers of the sea are in the area. Unfortunately, they follow Dagon! They are (roll 1d6):  1d4 clerics of Dagon,  1d3 priests of Dagon,  1d3 witches of Dagon,  1d4+2 cultists of Dagon,  1d4+2 fish men,  reroll twice. They are especially interested in slaying followers of sea gods, though they’re also happy to weaken any other human institutional resistance to invasion as well.
9. Thus unto seamen. The next NPC that a party member lies with turns out to be a fish man or woman! Apart from the tainted offspring, the party member only has a base 50% chance of getting out alive, modified by how well they did this time in Love and War (see Volume III).
10. Ice is awful! Ice freezes Dagon’s people! Ice must be abolished! A mob of Dagon followers (roll twice on #8 above) are attacking another group because of their association with ‘ice’. The other group is only about half their size- it seems that Dagon’s forces are always many- and are (roll 1d4):  Thrym cultists,  Skadi worshippers,  watchmen tasked with removing invaders,  Krampus devotees. The Dagonites have a base 66% chance of winning; that is, unless the party intervenes.
11. The party uncovers a valuable-looking but somehow unpleasant horn. It actually fits into a statue that some fools believe will bring about a golden age. Of course, it will instead transform into a Lesser Avatar of Dagon (1/2 AC bonus, HD, Dmg, & special abilities)! It will likely take some sort of mighty barbarian to defeat it by ripping the horn out again.
12. Look out! The disciples of Dagon have been busy. A minor invasion of the demon lord’s children occurs (sea people: whether fish men, sea raiders, or both), swarming the area at a rate of 3d6 per turn for the next 2d4 turns, likely making use of a Chain Teleportation spell to begin to replace the local folk forever. They might then say that it’s over, for such sea children seem no longer lost.
Sample 10th Level Cleric of DagonAlign: C
MV: 40’ (swim 80’ w/ mask)
Dmg: 1d6+2 (+1 harpoon- as spear) or 1d4+2 (+1 fish knife- as dagger)
SP: Dagon turn ability, mysteries, Foul Water Gout, Sea Seed, Refuge (divine tests), 25% chance of being seen as an obvious monster
1st level: (5+2) Create Water, Cure Light Wounds (x2), Protection from Humans, Remove Fear, Sanctuary, Divine WeatherD
2nd level: (4+2) Delay Poison, Hold Person (x2), Speak with Animal (fish), Spiritual Weapon, Warp WoodD
3rd level: (3+1) Animal Growth (fish), Locate Object, Prayer, Water BreathingD (reverse)
4th Level: (3) Cure Serious Wounds, Confusion*, Polymorph Self (fish)
5th level: (2) Chain Teleportation*, Feeblemind*
Possessions: +1 Scale mail, +1 harpoon, +1 fish knife, 165 gold pieces, 1 random magic item, plans for replacing the population of a nearby castle with fish men, two 1st level Dagon priest disciples, four Dagon cultists, pet bull shark (with a reversed Water Breathing spell cast on it if need be).
Next week: cleric encounters, Nergal!