- Nik Vinter
Yester Hill is a combat heavy zone that holds some interactions with Strahd and some magical items. Remember the place is huge and run your combat accordingly. Your players will arrive here most likely to find the gem; what they don't know is that the druids are preparing a ritual to summon Wintersplinter and asault the winery.
It's a short area that shouldn't be a big problem to run.
The druids are waiting for Strahd's blessing to start the ritual, when he arrives they're ready to proceed.
If ran as written the ritual takes one minute to complete, where at all times there must be at least one druid chanting.
Once completed a tree blight erupts from the statue present in the middle hill and starts traveling north, creating a path in the woods.
The blight is made with a gem from the winery, if you want to reward your players for creative thinking you can allow them to recover the gem from the moving tree (it shouldn't be easy though!)
The players can stop the ritual by killing all the druids in time, if they decide to do so you must underline that the stress factor is important, but you shouldn't tell them how much time they have exactly. If they come close but not enough to disrupt the ritual consider giving them extra time.
When the ritual starts is up to you, but I wouldn't wait more than the first druid death. If the druids are actually threatened then Strahd appears and they start the ritual.
You can also start it as soon as the players arrive at the hill, let them discuss their course of action and if they don't act fast enough let the ritual be completed.
Wintersplinter is summoned!
This shouldn't be a defeat for the party, but rather a new challenge, and depending on your party's level it might be a hard one (if the druids are not dead!). Consider making two separate encounters if needed: first the druids and then the tree, that at first simply travels towards the winery and doesn't attack unless attacked first.
This could make for an interesting chase scene, where the players have to defeat the druids while actually running behind the tree to catch him later and not let him destroy the winery.
Areas of the Hill
As usual, I'll address only the important ones without going too much in detail
If Wintersplinter isn't summoned the players will find the gem inside the Wooden Statue. Remember that players can be struck by lightning (makes for a good comedic relief) and that there are hidden druids almost everywhere.
An interesting thing to have in Barovia, especially if players know what it is. Other than that it's mostly useless if not for lore flavor. There are blights protecting it and a magical axe stuck in it.