*uses Ace14 definitions since is too lay to look them up myself*

The heart.

A reticence betrayed -> the heart, contained in the body, is taken away by Heartless

The ether decayed -> ether has an esoteric root and often relates to light, magic (as in a potion) or as a 5th element. Possibly representative of the nature of the heart (and it's subsequent disappearance)

Halcyon waylaid -> you are not really at peace when your heart is taken, no?

Erroneously portrayed -> this could be literal shape of a heart in the game not resembling the actual organ or referring to the metamorphosis that occurs when a heart is stolen (Heartless and Nobodies)

In conclusion my answer is: the heart. More specifically a stolen Heart.

EDIT: Relates to Xehanort insofar as it is "No heart"

Am I right?