You are standing on the edge of Aso Volcano's outer ring mountain, and the time goes back to an autumn morning in 1000 BC.
The sun was just rising in the east, its low-angle golden rays piercing the geothermal steam that pervaded the swamp. The first thing you'll see in the steam is the classic "hydra"—eight or nine huge, dark silhouettes wriggling in the vapor. It is an illusion formed under specific light and water vapor conditions after the reeds are divided by river channels.
Your reason tells you that it is an illusion of reeds and steam.
But your eyes will tell you another thing. Between the false snake heads swaying in the wind, there is a silhouette, moving against the wind. Instead of swinging with the reeds, it pushes away steam, leaving a muddy trail up to a meter wide on the water. As it passes, the entire clump of reeds shakes violently and then disappears into the water.
Your gut will tell you a third thing: That thing is moving in your direction.
And coiled on the rock not far below it is another real Titanoboa - its body is so thick that you would think it was just another rock at first. Its slowly raised head was directly under the phantom snake heads. The moment its letter was spit out, the phantom and the real body overlapped in the steam.
This is the most primitive source of fear of "Yama no Orochi". It is not a myth created out of thin air, but a holographic hell that is hard to distinguish between true and false, woven together by the light, shadow, steam and living ancient behemoths in the crater in specific seasons and years.
Intent: The era of the old gods comes to an end and the new king launches a challenge