Assassin's Creed Valhalla

This achievement requires you to complete three secret side quests, one in each of the first three regions of this DLC. Each one requires multiple playthroughs to complete, and on top of that, the reward for completing the Kaldstad side quest is extremely helpful. Therefore, it's in your best interest to get these done as soon as possible. 

Kaldstad
From the first encounter in this region, of the two chains you can take, take the chain to the left. Walk around the back of where the encounter is here, and you'll be able to jump across some ice platforms and into a cave. Follow the path up and through a smashable ice wall into a room with a sword you can Invoke to start an encounter. You'll notice, when doing so, that a cutscene shows a head lying on the ground, and then you'll fight an armless and headless body. Beat it, then on two more subsequent runs, repeat the exact same process. Each time, the enemy will gain more limbs and eventually a head. After the third time of beating this enemy, the quest will complete and you'll get a permanent favor, one that revives you once per run with half your health. 

Dokkerland
Immediately upon starting this region after jumping down, head into the building right in front of you and talk to the NPC to start this quest. At the very end of this region, before crossing over a big bridge toward the end, there is a big obvious building off to the side with a reward marked on your compass, and when you use Odin Sight while close, you'll see many NPCs marked in green inside. Head inside and kill the ten or so enemies, and open all the cages to free all the dwarves. Then, during a subsequent run, talk to the NPC at the beginning again. You'll need to fight off a bunch of enemies, and after beating them this quest will complete. The reward is a rope granting you access to the hidden gem merchant, which is required for Crossing Dókkerland (40G).

Nidheim
This quest line is the longest and most involved. First, when you get to the first set of bridges where you must choose to go left or right, go right. At the second set, again go right. At the end of this second bridge you'll meet an NPC that'll ask for your help in defeating a certain enemy. Right above him is a vertical area with a bunch of enemies. Kill them all and you'll kill the enemy in question in the process, which requires you to confirm the kill. On a subsequent run, the NPC will greet you right at the start of this region and ask you to get moonstones from the hidden merchant for him. Head back to the same area, going towards the gem merchant marked on your map. You'll likely wind up at a green glowing chest that's sealed with magic, and the merchant is around 30m below you. The entrance is found off to the side. Backtrack about 30m the way you came to get to the chest, turn toward the chest, and head off the left side of the path. If you look down, you'll be able to see some ledges below you (along with a lift to get back up). Jump down and there will be a small entrance into a cave with the merchant. Talk to him to get the moonstones, and when you exit the cave you'll automatically talk to the NPC and give him them. On another subsequent run, this time make a left at the first bridge fork. Just past the encounter on this side, you'll be able to find a small shrine of sorts with the NPC lying face down nearby, and a sword you can Invoke to start an encounter. Do so, and a werewolf will attack. You can hurt it, so instead just run to each moonstone (the things glowing extremely bright on the ground) and stand on one. The wolf will jump and get hurt from it. Move to each of the three until the werewolf is ready for you to just attack to finish off. Along the way, some regular druid enemies will spawn. Kill them off, then talk to the NPCs to finish this quest line finally. 

Once you've completed all three quests, in any order, this achievement will unlock.