The song is "The High Road," the hit from their self-titled 2010 debut (and only full-length album).
"The dawn to end all nights
That's all we hoped it was"
The song deals pretty directly with depression and suicide, but this line offers a glimpse of hope. The narrator of the song seems to helping someone with their troubles, leading to this optimistic phrase. This bright, new dawn will end all nights -- all dark and hidden troubles will be swept away by the promise of a fresh day.