As the group travels on the road through the forest, at first it's very dry. After some travel they come to an area where rain fell last night. The path is muddy and the trees are looking happier than usual.
There's a battle coming, if the characters want it. This is where the couriers carrying the Orb of Storms were ambushed last night. One of the couriers survived, and has climbed a tree sixty feet off the road, and there's a pack of wolves menacing her. But we'll introduce that slowly. First we'll tell them about the three dead couriers at the remains of the camp.
As the group travels through that area, they come on a camp on the side of the road. The camp has four sleeping bags and three elf corpses.
Investigation shows:
The corpses each have an elvish longsword, shield, chain shirt, and 25gp. They also have Empire courier uniforms and insignia.
(If a character finds money, you can avoid some friction by saying "25gp -- that's 5gp for each of your characters" to discourage loot-hogging.)
The corpses are drenched, as though the elves had been rained on a whole lot for days
The corpses were killed by wolf bites last night. It looks like the wolves attacked when they were asleep, and killed them before they could get out of their sleeping bags.
There are wolf tracks here, including tracks of one very large wolf. The large wolf went in one direction and most of the smaller wolves went in a different direction.
The wolves are about sixty feet off the road, sitting around a tree, looking up into the tree. There's an elf in the tree. It doesn't look like she's noticed you yet.
The elf's name is Alana, and when she notices the group, she calls out for help.
The wolves are not exhibiting "normal" behavior. Last night, a very large wolf told them to kill all the elves around that campsite. Smaller wolves obey larger wolves, so these wolves are sitting around this tree waiting for the elf to come down. They ignore the adventurers unless the adventurers act threatening. If someone uses *speak with animals*, the wolves explain that a big wolf told them to do this and they have to obey -- it's an "alpha" thing. If someone shapeshifts into a large wolf, the wolves will obey them.
If a combat happens (probably a combat will happen), the wolves will try to trip people and then attack with advantage against prone targets. A one-shot adventure should have an easy first combat so that players can get used to how their characters work, and this serves that purpose.
Alana wants to offer a quest to the group. ("Hey, are you guys adventurers? Do you take quests?") She wants the group to retrieve a very important case which she was carrying to Cedar Valley. The case was stolen last night by a very large wolf with glowing red eyes. The large wolf commanded a bunch of smaller wolves which killed her three friends. The case is covered in runes of containment, and the group can find it because there's a storm cloud over it. (Alana points at the storm cloud hovering over Freehaven: "it's over there right now.")
This is the exact same quest that the group is already on, except Alana is offering less information, and she wants the group to give her the artifact so she can give it to the mayor and not get blamed for losing it. Alana is also offering less money: 5gp per person, as opposed to 100gp or more from the mayor. This is an opportunity for the group to roleplay being nice ("oh, we're already doing that, don't worry") or mean ("you lost the Orb of Storms? How could you?").
Some groups have some things to say about the morality of deserting one's group while on watch. Alana's claim is that she shouted to warn everyone, recognized that this was a losing battle, and fled to save her own life. But, if she'd stayed and fought, perhaps her allies could have rallied and won the battle? Or perhaps more of them could have escaped? (In fact, Alana is correct; if she'd stayed and fought, it would have accomplished nothing but her own death.)
If the group forgot to ask the mayor about proper handling of the Orb, Alana can explain how it works. She also emphasizes that the group should not open the case, because that will let the Orb call dangerous storms.
Alana will not come along with the group unless forced to. She's not a particularly good fighter.
If the group tracks the large wolf that stole the case, they see that it heads for the bridge to Freehaven. At some point they notice the wolf tracks turn into tracks of a barefoot human.
Optional development: the evil energy that suffuses the area may cause the dead elves to reanimate as undead. (This is not even a house rule -- it's actually how zombies work in 5e!) I usually skip this when running a one-shot, but it can be interesting for a longer-form campaign.