Hello I can't help but answer this since the Pyramid Texts of Unas are something of a specialist subject for me (or were when I was studying Egypt).
@Nungali has for some reason changed the quote - to give a slightly different meaning as indicated in the translations you quote. So his meaning is specific to him (no less valid though).
I checked Sethe's original transcription for what is called Utterance 233 (lines 237/8) just to see what they were saying. Old Kingdom Egyptian is like a no frills version and so there is very little context or explanation in the text. But you can gain some context etc. by seeing where these texts were placed in the pyramid structure. In this case they are on the West gable of the sarcophagus chamber - and are part of a series of 'spells' against inimical beings. That is 'entities' or forces which act against the the process being undertaken in the pyramid (which is one of transformation and 'rebirth' as a spiritual being called an 'akh'.) The words are intended to be spoken to ward off these negative forces. These types of spells occur twice in the Pyramid once here on the West gable above the sarcophagus and once on the East wall of the akhet chamber (that is on the opposite end of the pyramid chambers so they reflect each other). So there are two instances in the process which takes place in the pyramid where negative forces have to be warded off.
Translated without interpretation the text says:
To be spoken: Falls down the serpent from the earth, Falls down the flame from the Nun (primeval waters), Fall down, slither away.
We have to break this down a bit to see what they were saying. First as I said it is a spell to be spoken out loud and its purpose is to ward off a negative force. One of the ways the Egyptians denoted negative forces was by a state of inversion. So there is a right order for things (ma'at) and there is its invert which is negative/evil. So the first two lines describe a negative state which has arisen. That is the serpent has fallen and the flame has fallen. This describes a negative state. Why because serpents like the uraeus (Egypt. Iaret ) literally means 'risen up one' and obviously also flames rise, they don't fall. I think also that the word flame or fire which has the determinative of a torch is referring to light rather than heat. So it is referencing light = awareness which has fallen back into the primeval waters which are dark (kek).
So if you imagine that the right (ma'at) state of things would be a risen serpent (like the cobra on the brow of the king) and light emerging from darkness. But because of the interference of inimical beings/forces the opposite has happened and this needs to be addressed. The way this is done is to turn back the negative force onto itself. Like saying 'you made these things fall, now you fall!'. This symmetry is very Egyptian.
So the last part of the spell is saying to the negative being 'You fall! and slither away!' (or you fall slitherer)
In fact the very last word 'sbn' (slither) gives us a clue to which category of beings is being addressed - it is that of the Sebau fiends ... or Slithering beings. This refers to forces of dissolution or perhaps wasteful energy loss. Like for instance lustful thoughts and the like which drain energy.
So my rather free translation would be:
"You who made the earth serpent fall, who made the flame from Nun fall; You fall and slither away!!!"
