A glabrezu moved into Unicorn Woods several years ago, and has a plan for gaining power in the Abyss by creating an army of half-fiends, himself being the father, of course.
The first phase of the plan involves, naturally, unicorns. The half-fiends are physically identical to normal unicorns. The counties surrounding Unicorn Woods are home to a sect of female druids, whose coming-of-age rituals involve following a unicorn for three days without distrubing it. The glabrezu intends to make these druids the mothers of the next generation of half-demons, and is using the unicorns to lure the druids to him.
Once a group of druids-in-training disappear after following the unicorns, can the PCs track them down and save them from an awful fate? And how will they know which unicorns are a threat?
A mysterious white worm disease is going through the upper class neighborhoods of the city. The patients are first afflicted with a loss of energy and then slowly they begin to become paler and eventually die or go mad.
World famous Dr. Nedremus has been attending to some of the latest cases and has declared them White Worm's disease, an affliction spread outdoors during the summer and picked up from a parasite that relishes hot, bright summer days.
The true answer is the doctor is under the influence of ancient vampire who's lair is deep beneath the city in some ancient ruins. Employing gaseous form the vampire cruises through even the smallest of sewer openings that serve the city's richest districts.
The PCs must discover the answer while the rich take the good doctor's advice and stay in dark secluded places to avoid White Worm's disease.
Homecoming.
The PCs are called home for a special reunion with a long lost uncle. The party goes smashingly and the PCs leave content that all is well at home. A few levels go by doing some adventuring and the PCs forget all about the happy episode.
They receive a message that the church in their home village has been destroyed. The church asks them to check it out.
The PCs immediately figure out something is very wrong in the village. Cattle are not being tended to, the fields are a mess, and the graveyard has been disturbed as well. That night they are assaulted by a group of lesser undead.
What's going on?
The long lost uncle was in fact a vampire. (Thus, he was long, long lost.) Further, he has not left the village, but instead has taken up residence under the graveyard in some ancient crypts. The Vampire has converted many of the villagers into spawn and created several undead to serve him. He mostly hunts in other locations these days, but he realizes the rapidly deteriorating village and destroyed church will soon call attention to the situation.
If/when the PCs destroy the vampire spawn, the vampire will move on rapidly. Perhaps remaining a BBEG in an ongoing campaign - preying on isolated villages until it gets too hot and he moves on.
The Set-up
An evil king from a neighboring nation has just died. His son through an envoy has made a peace gesture and invited the PCs' patron to the funeral. The PCs' patron is highly suspicious of this diplomatic move and portents delivered by his clergy have not been favorable. The Patron is intensely curious and does not want to miss an opportunity for peace.
The patron calls on the PC to go to the funeral in his stead. One of the PCs (preferably a rogue or bard) should go disguised as the patron. Few others, if any at the funeral would recognize him. The PCs are authorized to speak as the patron and told to bring any formal peace initiatives back home. They will be rewarded accordingly.
The Real Deal
The evil king has died, but was assassinated by his chief priest of (insert evil deity of your choice). The evil priest has now taken on the persona of the king's son and wishes to wage war on the PCs' patron. The funeral invitation is an attempt to sever the head of the PCs' patron's nation before launching a surprise attack.
Execution
The PCs will arrive at a funeral wake feast for the departed king. The king's son (actually the evil cleric) invites the PCs to view the body with him and pray. He goes totally unarmed to show his good will. The casket is kept in a back room. When the casket lid is removed, 10 vargouilles fly out to attack the PCs. The cleric casts hold person on various PCs to allow the vargouilles to use their kiss attack. The cleric then flees out a back secret entrance forcing the PCs to deal with the monsters.
Epilogue
If the PCs survive, they learn evidence of what has happened and that the evil kingdom is planning a massive invasion of their Patron's lands. They must race back to warn their patron and help prepare the defenses. (Have your new Miniatures Handbook ready.) Part of the evil cleric's attack plan should involve sending in flocks of vargouilles!