As an aside, most heroquests are not about changing reality, or rewriting major myths – for what it is worth, I find such heroquests are not all that interesting. They are about discovering something new – in the mythology or in the characters themselves. Here’s a really basic one: The Earth priestesses are all in a worry this year as the winter solstice has passed and the Grain Goddess has still not begun to awaken. If she does not return, there will be no spring planting, let alone a fall harvest. Everyone is worried. Omens and divinations are taken, and it seems she is trapped in the Underworld by a “Demon Lord with a Bone of Power”. The Earth Priestess writes names of those in the local community on shards of pottery and puts them in a cauldron. She picks them out in a ceremony attended by all- it is your characters who are named! She says that you all must enter the Smoking Cave (located near the Cinder Pits) and descend deep within until you pass out of this world. She gives you an emerald crystal with two powers – it is a spell reinforcing crystal of 4 points, and it glows brighter when held in the direction of the Grain Goddess!
On the GM side, I would make a little mind map of the Otherworld, and a few mythic locations and obstacles that they might overcome. So there is some sort of Chomping Maw at the bottom that they need to pass through; there’s a Plain of Grey Ash, populated by red skinless trolls that breathe fire; there’s the Path of the Dead (but don’t follow them, they are going somewhere else) where they might get to talk to an old enemy or an old friend; there’s a Ball Contest with the Striped God where the characters need to wager their arms if they want to get the direction to the Demon Lord; and so on. THere’s even a Red Woman who offers to show them the way. Maybe they fight, maybe they talk, maybe they sneak, maybe they make bargains – all are options! In the end, they confront Monster Man, who has the Grain Goddess imprisoned as his house servant, but he’s too strong in almost everything they try. But he’s got a hideous wife who is surprisingly kind to the characters. She’s willing to tell them her husband’s weakness, for a price…. That kind of stuff. In the end, the characters interact with a number of archetypes which they might identify as various deities or entities. They get some neat powers or heroic abilities, get some increase in their Runic abilities, etc. They bring back the Grain Goddess and their community is filled with happiness. Or they don’t and they get banished!