Before the extinguishing of the light
before the coming of the Darkest Dark
it arose as mighty sight
til the flood did render it start.
???
I think that the riddle talks about Numenor, and possibly the answer could be Atalante or Akallabeth, because this is the name of Numenor after the downfall. Another possibility is Meneltarma because it’s the great mountain of Numenor. After the downfall everyone thought that Meneltarma' peak could be above sea level, but nobody found it.
Update: not Almaren, Numenor, Hyarmentir, Ezehollar, Two Pillars, Utumno, Numenor, Meneltarma, Tol Eressea, Avallone, Taniquetil
Further Update: Scott Nolan thinks it might be Thangorodrim.
1st Failed Attempt: Thangorodrim
This doesn't seem likely to be Numenor-related, since is says "Before the Extinguishing of the Light". Sure, that could refer to the light of Numenorean civilization, but it more likely refers to the extinguishing of the light of the two trees, before the start of the First Age. And the "Darkest Dark" is probably Ungoliant's "unlight".
Therefore, this must be a mighty sight which arose before the rising of the moon, and which was destroyed in one of the two floods Tolkien describes: that which sank Numenor, or that which sank Beleriand.
There is considerable discussion of this riddle on the forum along with many other failed answers, under: https://forums.mepbm.com/t/insane-fa-riddle/15005