Assassin's Creed Valhalla Myth Quests become available after building the Seer's Hut at Rank 3.
To unlock Myths quests, you first need to raise your Colony to Rank 3. This is actually quite easy to do early in the game, as Rank 2 is a bit of a giveaway. Ranks 1-2 only require two buildings to build and you will have the necessary materials after the tutorial raid. To go from rank 2 to rank 3, you will need to build 4 other buildings. Two to three raids will net you the necessary supplies and raw materials, although you can skip a raid by completing one of the first arcs in the Alliance area to increase the rank of settlements. Essentially remove raids in the first two areas that are at your power level, or a few and the area arc.
Once you've reached Rank 3 in the settlement, you'll unlock the Seer's Hut, but you'll need to build it first before embarking on the Mythic Quests. It will cost you 800 supplies and 60 raw materials. Once you provide Seer Valka with a place to rest her tired head, she will offer you a quest to unlock the Mythic Quests. She'll need a few plants to brew a potion in Eivor, and the items from the first batch are all nearby.
Once you've handed her the herbs, she'll brew the weird brew, and then you can drink it from inside her hut to begin the first Myth Quest in Asgard. Yes, that Asgard. I'll avoid going any further for spoilers, but you want to complete the myths quests in Assassin's Creed Valhalla. They fit in perfectly with the main story, and they're just a blast to boot (assuming you enjoy Norse mythology).
You can exit a Mythic Quest at any time (after completing the first stage of the Intro Quest), and it is recommended that you have power level 90 for the first segment. There's a second segment, and you'll want to be about double that before you jump in. Otherwise, take advantage of the Mythic Quests: they're definitely worth the time to unlock the Seer's Hut.
