The Second Sin is the name of the pact that Arthe made with The Stranded Light to gain the ability to destroy The Soul of Glass during the Shattering.
In exchange for The Break of Dawn, a song to able to destroy another voice, Arthe promised to kill the other Noble Gods so that they would never again harm the mortals, and then to kill herself to atone for using the song.
Walk in the shadow,
at The Break of Dawn.
I’ll close the eyes
of brothers and sisters.
I surrender my child
until I close mine.
The Queen promised,
at The Break of Dawn.
After the song,
she watched the sun set.