Note, this is going to be a mess, trying to just throw something together since the quest won't be here long.
The start of the quest chain for Horde is in Orgrimar, talk to Chieftain Grimhowl in front of the bank. On the Alliance side look for Magistrate Dunwold in the Trade District of Stormwind by the tree. They give you "A Frost-touched warning" and send you to Winterspring.
Go to Winterspring and find the Scout Wisperwind in a cottage by the Starfall Village there. Turn that in for 50 snowballs.
She gives you another quest to go into a cave behind the village. Inside the cave is a riddle. You need to click the runes inside the cave in a specific order to continue. When you click the rune and confirm you should get a shock. If it turns you into a sheep you got it wrong and need to start over. You have to talk to Keeper Zerathis to finish the riddle, but not to start it.
Pictures here, with red marks where the rune is: https://imgur.com/a/Y719y5P The pictures are in the order of activation.
This is the order to activate the runes in:
Void and light (Void monster)
Birth of the Titans (Star skeleton guy)
Order Against Chaos (Stone titan guy)
Naaru and the Light (Naaru thing)
Fall of Sargeras (Demon) (Upper floor, behind Grum Redbeard, hard to see him)
Birth of Azeroth (Planet)
The ancient guardians are humanoids, so you can control them (Hex, Polymorph) but when they snap out of it they can pull you back through the walls. They are about as tough as heroic bosses.
Then go to Keeper Zerathis and tell him you solved it. (Near the planet rune, with all the example runes)
Go back to Scout Wisperwind and turn that in for 5 Great Feasts.
Outside now you will find "Spirit of the Winter Veil" with another quest. Go talk to the citizens of Everlook.
Talk to 6 "Everlook Revelers" and get 6 Winter Veil Tokens. Head back for 10ea Runic Mana and Health potions.
It gives you another quest to get 10 Crystallized Shadowfrost Energy from the frost-touched creatures in the area. Thankfully it is a group drop, so team up.
These mobs have a crazy AOE distance, 100 yards. So careful not to get AOEd to death from the one someone else is fighting ont he other side of a hill. They are about as tough as Heroic dungeon trash. Some have stronger AOEs than others I think.
Turn this in at the spirit for another 10ea Runic Health and Mana potions.
Then take the next quest to find an artifact in the lake. It's a tall blue glowing crystal, hard to miss. Click it and head back for a bunch of level 8 scrolls.
The spirit gives you another quest to go back and meet it near the crystal in the lake. (It is slightly NW of the crystal) Turn that in for 2x level 80 flasks of your choice. Then you fight the boss.
NOTE! Be very close to him when he starts the fight, he puts up a damaging wall of ice when you start the fight. If you're caught outside this you're screwed. I don't think he has an aggro radius and just waits to be hit.
He spawns frost on the round so keep him moving, I think this frost lasts the entire fight and never fades. Also spawns frost orbs that fly around and hit hard, but they move kinda slow. Towards the end he spawns elemental adds.
I beat the boss with 14 people. a couple of deaths, but you can run back, res, and rejoin the fight.
The quest suggests 25 people, I would say 10 as bare minimum.
FYI you can keep the quest and wait to help others fight the boss. You are not locked out of fighting the boss again until you turn in the final quest.
Rewards are an account-bound Murloc costume and a Glacial Bag. I hear you can repeat this each year.
Brought to you by <The Flat Azeroth Society> of Lordearon Horde. "Keepers of the Chill." ;) Thanks!