Once you've unlocked the Fiery Planes section of Mythopia in 's expansion, you'll automatically unlock a quest, bringing you one step closer to completing another Mythic Trial.
While you'll have to complete the one in the Elysian Fields first, you can complete the remaining two trials in any order. The remaining Mythic Trials work similarly to the one that allowed you to unlock Hades in .
First, you will need to complete a quest unlocking the portal, which, for the Fiery Planes, includes upgrading your Watering Can so you can water the flames and reach the lever.
Once you've done that, enter the portal to begin The Promethean Flame quest. When you enter the Mythic Trial, lower the lever in front of you to allow you to cross the bridge and speak with Hades.
He'll inform you that, for this trial, you'll need to help him light three torches. Though it sounds different, it'll be the same process as the other two Mythic Trials since you'll just be pushing carts to move torches until they're in the correct position.