You've played Castle Mistamere a hundred times. Time for something familiar but different. Apologies if this has been done before. Searching for Aleena related posts turns up a lot of references...
To Save Aleena
by David TauriainenThe scene: Threshold's central square. Woodsmen and visitors to town are bustling in front of the town hall, where their axes and other large weapons have been kept by the town guard during their stay. A general murmur goes around the unusually crowded square, and the player characters eventually hear that a bounty has been placed on the head of a locally infamous wizard, Bargle, for having recently killed Aleena, a young and well-liked cleric.
If the PCs ask around, they will initially be told that no one knows where Bargle is, but groups are forming to head out into the wilderness areas. If the PCs ask earnestly enough (or buy enough rounds of drinks at The Black Jug Tavern), they will learn about a group of adventurers and peasants heading up to Castle Mistamere, where they think Bargle might be hiding. Assuming they decide to join this group which is meeting on the north side of town, they will be intercepted by an acolyte from the Church of Karameikos who will ask for their help in another matter (see below)
If the PCs decide to ask for clues at the Church of Karameikos (an obvious first step if they aren't throwing money around at the bar), they will be greeted warmly by the clerics, although they will ask for the PCs to be respectful due to Aleena's passing, and might let slip (although it is not a secret to anyone from Threshold) that she is the niece of Baron Sherlane Halaran, who is also the local Patriarch of the Church of Karameikos. The clerics seem sad, but not distraught as the villagers are. They do seem nervous however. Once pressed (or when the PCs are leaving), the clerics ask for the PCs' assistance.
It seems that Baron Halaran, the only cleric in Threshold able to raise the dead back to life, has left several days ago down river on a trip to personally bring a tribute of the finest wood and furs of Threshold to the Grand Duke, as well as tithes for the Church of Karameikos in Specularum (and a large amount of secret newly minted silver coins for the Ducal treasury - known only to Baron Halaran and a few select elven guards). The PCs are needed to race down the river to bring the Baron back to Threshold in time (Raise Dead has a time limit measured in days, and the Patriarch does not have access to Resurrection which measures in decades [or Raise Dead Fully in OD&D, which can measure into months).
The clerics have no boats, so the PCs must procure means of their own (slowing their pursuit from the outset). The PCs have an advantage in that Sherlane and is entourage are riding a slow moving barge, however monsters and perhaps natural features of the river, roads, or weather prevent them from moving at full speed, and Sherlane does have a head start. If the PCs are dirt-poor, a free ride on timber-raft bundles can be had if the PCs promise to get them to their destination in Krakatos, but this is no faster than the Baron's speed, and they will have to use their meager earnings for delivering the timber as fare for the final boat ride to Specularum. They could also race via land from Krakatos; the roads are far less dangerous this far south.
If the PCs have enough money in Threshold, they can get a ride on the weekly trade ship to Kelven (20silver/person, 2-day trip, but must wait 1d4 days for the boat to arrive). From Kelven, there is a daily boat to Krakatos (10silver/person, 2-day), and two daily boats to Specularum from Krakatos (3silver/person, 1-day).
Ingenious, lucky, and rich characters might think to rent a magic carpet ride (2 person max) from Cardia, a local elven entrepreneur, seemingly bypassing a large portion of the adventure until an aerial battle ensues with a wandering flying monster.
If the PCs reach him with time to spare, he will seem torn between his duties as Baron and Patriarch of Threshold, and his love as a father. If the players do nothing to change his mind, he will dictate letters of apology for the Elvenguard to present to the Duke and Lord Olliver Jowett, the Patriarch of Specularum, and return home in the fastest way possible. Even if the players have to chase around Specularum and don't find the Baron until two days are left, he will thank them curtly and use a Word of Recall to return home instantly, giving no second thought to his duty. If they pass beyond the time limit, Aleena will still be saved by Sherlane taking her down river (asking the PCs to help guard the retinue) to the Patriarch of Specularum, who will raise her. The primary purpose of this adventure is to explain Aleena's survival, after all.