Hello everyone.  I am looking for help in tying up my campaign.

I set off several months ago to write a very ambitious campaign, set in an artic barbarian culture with their own money, gods and weapons.  The basic goal of the campaign is to survive a particularly brutal winter.  I wanted a bleak, austere and spartan world.  Besides gathering food for their starving village, there is a significant subplot about traders.  The traders have been coming in ever greater numbers over the previous years to obtain “kopai” a metal used in the rest of the world as a rare currency that is found in this region.  Dwarves have started to come to the region and have been secretly delving into caves near the coast; the locals surmised they were freezing to death as they never saw them after they got off the trading ships.

We have played through three sessions.  The PCs have found some food for their village and have determined that the dwarves actually established several mines.  They have found a few small kopai rocks and fought some local carnivores (wolves and bears).  Lastly, they found that a merchant ship overwintered at the mouth of a river as it got iced in.

There is also a meteorite far off into the tundra that can actually be mined for fire weapons; although they know of its existence, they do not know the true significance.  There is also a known hot spring, an oasis of sorts, far away that they have yet to explore.

How do I wrap up the adventure with a bang?  I though about having a major monster at the hot spring.  I initially wrote that a kraken had swam there from the ocean but am not in love with that.  I considered a frost giant(s) or white dragon taking over one of the villages as well but that seems a bit cliché.  Another idea was to have a number of hot springs, which a mage(s) controls, perhaps dropping the temperature elsewhere for some nefarious motive.  I need some fireworks!  My players prefer battles and have an evil slant.  Thanks for your help!

One way to wrap up the adventure is to have the dwarves accidently unearth something buried deep beneath the ice, or connect it with the meteorite that's landed. The revelation could be an Illithid (mind flayer) ship that crashed and froze in the tundra. Or a Remorhaz that waits next to the hotsprings for prey to wander by and then ambush them. Or perhaps an entombed Nightwalker has been released and seeks to destroy all life in its wake.

Perhaps a Devourer has been sent into the tundra with the order to harvest as many souls and build an army of undead, that will make its way to the villlage.

Piggy-backing off the Illithid idea, you could have the hotsprings actually be the hunting grounds of an Adult Oblex that was created and experimented on by an Alhoon. With an Oblex, you can play up the psychological horror, playing with character's memories, confusion, etc.

Perhaps one of the elders within the village is a Bheur Hag and has been manipulating the weather to bring a harsh winter upon the village to forment dispair amongst the people and incite violence. 

Those are good ideas.  I had a Remorhaz in the dungeon already.  I have set up a "random encounter" with a "juvenile remorhaz" that was already being attacked by a party of dwarves so it wouldn't toast my low level PCs.  Given their inclination, I don't know if they will help the dwarves or sit back and wait for the dwarves to die so they can rob their corpses.  

I like the idea of the dwarves unearthing something unpleasant and the Bheur hag is genius.  

Thanks!

I thought some about the Bheur Hag.  

The party is about to enter a friendly village that they have already had contact with.  What if they find a human corpse near it, stripped of the flesh...  Maybe one of the warriors in the village recently had a rendezvous with a mysterious, young maiden.  He is obsessed with seeing her again and keeps fingering a colorful broach she gave him (actually a hag's eye).  

They will need to return back to their home village afterwards (for reasons too complicated to explain here).  When they return home, the village is starving.  Although they had enough food to get by in the party's recent 10 day trip, people started hoarding food to themselves and several of the underground food caches were plundered.  Investigation shows that they were dug up with shovels, not spoiled by animals.   (Memories of COVID anyone?)  This led to the death of several villagers.

Good?  Bad?  More details?

So we resumed last night after a Christmas break.  I gave them what I hoped were several good clues as to what else was going on besides just a cold winter.  They came upon a body of a man from another village.  The meat had been stripped off the extremities.  Notably no injuries to the torso (the hag cast held person and then ate him alive).  A clever player asked about footprints in the snow-there were only footprints from the victim as the hag flew in and flew off.  A small amount of money and food was unspoiled in the NPCs backpack (not robbery or animals).

They then delivered their food to their home village; survival through the winter is one of the main themes.  It was the middle of the night so they went to sleep in their homes but were awakened early the next morning as many villagers had crowded around the sled and had begun looting it.  This was atypical behavior as the chief had always distributed food in an orderly manner before.  They chased down a few leads and among other things found that a man had been missing for a few days but turned up.  He was really happy and had a new ring (jewelry is very rare in this world).  He had a rendezvous with the hag who had disguised herself as a young woman to become pregnant.  The ring is a hag ring.  He simply told them he found the ring on a hunting expedition. 

They then had a few unrelated battles with local fauna.  

Any ideas on how to give more clues as to the identity of the real source of their problems?  I plan on having a few more victims in the tundra, under similar circumstances.  There is one last village in the area they have not gone to and I am planning on making the village a ghost town, with numerous corpses (the hag started there before traveling to their area).  Other ideas?  They are also going to really struggle to kill it.  Very few missile weapons and few offensive spells.  If they can't destroy the greystaff first, they are not going to make it.