There are 5 in total.
2 for each and one when
Mission one:Second Hand Soul
This is activated the first time the water drops at the lake of the nine.
You are charged with finding the ring of Brok's alchemist friend. The quest is fairly simple when you reach the Völunder Mines. I won't post any spoilers, but there is a hard fight if you haven't put some effort in to building a Kratos that you can really deal some damage with.
Mission Two: Deus Ex Malachite
Brok has another missions for you, this unlocks when you complete the first mission and the water in the lake of the nine drops a second time.
Talk to Brok in any of his shops and he'll want you to meet him at the Landsuther Mines. The entrance is on the lake of the nine, west of the Muspelhiem Tower. He wants you to find Andvari's hammer. There are a lot of od Dark Elves and an particularly hard Dark Elf Captain. The Traveler is also challenging on harder difficulties.
NOTE: Once you have the Hammer and return to the first room, make sure to drop right to the bottom of the hole you created to begin with, take the lift on the left hand side of the map in the water. Follow the path up to get a bunch of resources from the high end chest and a scroll of lore for Atreus.
Mission Three:Fáfnir's Storeroom
Sindri offers this quest to you after your visit to Alfheim.
There are Five doors to unlock during this mission.
You won't find what you're looking for (surprise surprise), but once you complete it you'll recieve the Talisman of Betrayl for Kratos.
At the end two Fierce Orges will appear. Target one at a time and always keep an eye the other. Before taking the Orges on, kill the Nightmare Spawns that come along with them.
There is also an optional boss on your way out of an Forest Ancient. I'd recommend taking it down, as it counts towards an Labor.
Mission Four:Family Business
This spawns after you complete Sindri's first quest and get past "The Sickness" Main quest.
You're tracking down the son of the bandit from the first mission.
Northri Stronghold is down the Northri Gorge, this is just passed the Ruins of the Ancients in the northern side of The Lake of the Nine.
Sindri meets you outside, make sure you talk to him. Missions Five (also has a Dragon Favor running through it)
When you get to the Stronghold you'll be greeted by some Hel-reavers. Make you way to the front door and discover you need two more pieces of keystone. Take the left path first and find the stone (this part is easy, you won't need any help.)
The return to the door and take the right path. Down this path is the Dragon. Destroy the totem infront of you as you climb down the hill. Defeat the hel-reavers. Follow the water around to find the second and then return to the Dragon. The third is blocked by a door. You can make Atreus do all the work here.
There is a stumo nearby with the explosive crystals nearby (Darksiders anyone?), throw one of these between the two on the left. Now, get Atreus to hit that one with a electrical arrow and quickly hit the Ore on the right.
This opens the door.
Use your Axe to freeze the poison in the middle, Use your Choas Blades to dispatch everyone, The Dragon is pissed off at this point and starts attacking everything, including you. So be careful of its attacks. On the pedestall is the third key part and behind that, the final shrine to freeing the Dragon. Pick the part up and destroy the totem. Return to the chain and release the Dragon, then back to the door to continue your quest.
Each stone you put in triggers a wave of enemies, so be prepared to fight.
The rest of this quest is simple, so I'll say no more as it may spoil the story of the whole thing.
NOTE: upon entering the Stronghold, take a detour right for an Jotnarr Shrine.