I can't help but think of this song as Rogal Dorn's perspective.
(my interpretation contains Primarch incest)
Daddy, please hear this song that I sing
In your heart there's a spark that just screams for a lover to bring
A child to your chest that could lay as you sleep
And love all you have left like your boy used to be long ago, wrapped in sheets warm and wet
Rogal Dorn has a vision of Malcador or Erda bringing the Emperor and infant Horus. "All you have left" - now the Emperor is on life support and no longer lucid.
[Sister] please with those wings in your spine
Love to be with a brother of mine, how he'd love to find
Your tongue in his teeth in a struggle to find
Secret songs that you keep wrapped in boxes so tight, sounding only at night as you sleep
Rogal Dorn asks a dead Sanguinius to take him to the afterlife to meet Horus. Sanguinius was romantically/sexually involved and knew things about Horus that Rogal Dorn didn't.
And in my dreams you're alive and you're crying
As your mouth moves in mine, soft and sweet
Rings of flowers 'round your eyes and I'll love you
For the rest of your life when you're ready
Rogal Dorn has dreams where the people he loved the most are still alive and able to be emotionally vulnerable around him. The dreams about Horus are the most painful.
Brother, see we are one in the same
And you left with your head filled with flames and you watched as your brains
Fell out through your teeth, push the pieces in place, make your smile sweet to see
Don't you take this away, I'm still wanting my face on your cheek
Horus died violently, trying to kill the Emperor. I think about the artwork where his face is lit up red, like the flames of hell. Even though it ended so badly, Rogal Dorn still treasures the memory of Horus' face on his cheek. He's mentally putting that shattered violent image back together and beautifying it thinking of how things could have ended differently.
And when we break, we'll wait for our miracle
God is a place where some holy spectacle lies
When we break, we'll wait for our miracle
God is a place you will wait for the rest of your life
The Emperor is considered a god but has to wait on the Golden Throne for the rest of his life. Rogal Dorn doesn't believe that a miracle is going to happen and everything is going to be fixed. Horus is dead and gone and there isn't an afterlife for him to go to because his soul was destroyed.
Two-headed boy, she is all you could need
She will feed you tomatoes and radio wires
And retire to sheets safe and clean
But don't hate her when she gets up to leave
Sanguinius loved Horus but the time came to leave him because he turned traitor.