Deathless Amon was an ancient myth told amongst the Kaim Tribe in Amon: The Darkside of the Devilman. It foretold of an ancient creature known as Amon who was considered to be the first demon to be born into the world and hero of the demon race, and how he grew more and more powerful which worried the gods. They murdered Amon and trapped his soul in a massive orb in the sky which was called Deathless Amon (The Orb may have been the Moon). However on the nights when Deathless Amon was at its biggest, and if you were to get to the highest point in the land "Amon's Ladder" would come down from the sky and if you could climb the ladder you would be granted Amon's power and soul. The legend was told through generations, it was told to Sheena and the "White Monster" by Kaim the last of the Kaimu tribe.
The White Monster eventually "climbs" the ladder joining with the Deathless Amon to becoming the Amon who would eventually merge with Akira Fudo in Devilman.