Lyric discussion by God_of_Drums 

Cover art for Death Whispered a Lullaby lyrics by Opeth

I also believe that this song may be about a child serial killer. My interpretation:

Verse1: The fireflies circling could be the souls or spirits of the dead that cannot leave this world. The lost souls that hide are the souls of the children that were buried in the woods. The men returning trying to find some peace of mind is the search party for the missing children. They still cannot find the children.

Chorus: This could mean simply, sleep my child all will be fine. And then he'll kill her once he reaches the woods. Or this could mean sleep as in an eternal sleep, or simply Death.

Verse2: The shadows moving under the fog could be the usual stir of animals and nightlife in the woods at night. The killer tells the child not to be afraid and take their hand.

Or, this could describe the ghosts of children who were murdered previously which supports my interpretation of the second half of this verse.

Into the dark there are eyelids closing. This could mean that the children's spirits do not want to witness what they had to go through themselves. They cannot do anything to protect the child, for they have already been buried in the sands (quick sand perhaps?) so they try to ignore the horrible scene.

2nd Chorus: This could mean the same as the first. Or this could mean that the killer is now doing the terrible act telling the kid to simply accept death. This supports my interpretation of the next verse.

Verse3: Speak to me now and the world will crumble could mean that the killer will be reminded of previous children he/she has murdered and it could prevent them from finishing what they've started. If the child is freed or escapes the killer's world would crumble cause they would be recognized and hunted. Idk, but that's my thoughts on that line...?

Open a door and the moon will fall could represent how if the child just accepts their death the moon will fall and sunshine and happiness will be back to greet them again, eternally?

The ending is difficult for me.

So, the killer tells them to go to their dreams and forget it all. Does this mean the child is killed, or set free cause the killer realizes how evil he/she is?

I don't think that's the meaning the writer intended, but I really like it, and think it gives a sinister twist to an already dark song. Nice interpretation.