Amnon was a prophet of old according to the early version of the legendarium in The Book of Lost Tales. His most known prophesy was "Great is the fall of Gondolin", which was quoted again by Turgon, King of Gondolin, at the time the event took place.
Other versions of the legendarium
In the first version of the Doom of Mandos appearing in The Book of Lost Tales, it is a servant of Vefántur who proclaims, among other misfortunes, the fall of Gondolin; and the Noldoli remembered those words as the "Prophecies of Amnos" (Amnos being the place where they had heard them). Later, Tolkien wrote a couple of notes referring to the "Prophecy of Amnon", with the words "Great is the fall of Gondolin. When the lily of the valley withers then shall Turgon fade", but it is unclear if here Amnon is a place or a person. As Christopher Tolkien suggests, the prophet Amnon could be the servant of Vefántur that proclaimed the doom of the Noldoli.
In the Bible, Amnon was the wicked eldest son of David. Tolkien was surely aware of this character (as he was a Catholic), and it is curious to speculate on his particular choosing of this name, as it was in his earlier works.
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Book of Lost Tales Part Two, "III. The Fall of Gondolin", p. 184
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Book of Lost Tales Part One, "VII. The Flight of the Noldoli", p. 167
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Book of Lost Tales Part One, "VII. The Flight of the Noldoli": "Notes and Commentary", p. 172