Check out Volsung's The Shadow
on AO3—Basically it's a what if-story where Luke and Vader discovered their relationship to one another in ANH rather than in ESB. It actually follows the plot of ANH relatively closely up until ESB, and then has a massive divergence from when Luke fights his father onward.
The entire time I read it, I could only think about how well the Florence + The Machine song Cosmic Love
fits this fanfic, particularly this lyric:
And in the dark, I can hear your heartbeat/I tried to find the sound/But then it stopped, and I was in the darkness/So darkness I became
The stars, the moon, they have all been blown out/You left me in the dark/No dawn, no day, I'm always in this twilight/In the shadow of your heart
made with AzaleasDolls and procrete