It's true that "wraith" rhymes with "faith", but if the composer of that song didn't mean "wraith" literally they could surely have thought up another rhyme. I think the song means what it says. The Wraiths of Andelain have in the past betrayed the Land, no matter how harmless they currently seem. You wouldn't think such a thing based on Atiaran's reactions, but Atiaran wasn't the best possible fount of knowledge, as she herself repeatedly said. For example, Atiaran seemed to think that the Vilesilencer defeated the Demondim.Seven hells for failed faith,
For Land's betrayers, man and wraith:
And one brave Lord to deal the doom
To keep the blacking blight from Beauty's bloom.
Creators's children = stars = Elohim = Viles = Wraiths. (More properly, I would need some group theory symbols here, since I'm not saying ALL the Elohim are Wraiths of Andelain...) An important clue is to look where star imagery appears. The similarities are striking. I think they are all connected. In Runes we got a big piece of evidence when we learned that the Viles were immaterial creatures, and that both Viles and Elohim are capable of what sounds like instantaneous travel.
My theory is that Loric Vilesilencer, who notably is not called Vileslayer, found with the Demondim's help a way to reduce the immortal Viles into a state as near death as possible, much as Covenant later did with Lord Foul. But in this case, the Viles were unable to recover their power on their own and the only thing they could do was to hang around and observe much like the Dead later on.
But then Covenant came with his ring and offered the Viles a chance to replenish their beings with wild magic. (I suspect the Viles may have foreseen that Covenant would come.) Besides inducing despair in Covenant, the slaughter of the Wraiths may have been about preventing the returned Viles from becoming a complication in Lord Foul's plans. We know that the Wraiths of Andelain were not mindless creatures, only mysterious and alien. We have seen them behaving in a way that presupposes some intelligence.
By the way, I have been thinking that the Ravers may be related to the Viles somehow. Either they would be particularly accomplished and traitorous Viles (making their long-forgotten mother, if they have one, very possibly Diassomer Mininderain) or they would be the original Vile-spawn, whose production was stopped when even the Viles saw the ethical problems inherent in the design and decided that further Vile-spawn should possess only people who no longer needed their body. We know that Loric's krill was effective against a Raver, but this might not necessarily mean anything much.
However that is, there is the angle that ur-viles ate Wraiths and Nom ate Sheol. But the Worm ate the stars which correspond to the Elohim and the Elohim didn't pass utterly. Perhaps the ur-vile that is helping Linden is the one that fought Covenant at the Celebration of Spring and Wraith residue is responsible for its change of mind. I would really like to see what's Nom up to and I wonder if the Dead or the souls of living people can be eaten. (I'd hazard yes, at least for the Dead. I wonder if something like this happened to Pietten - there were ur-viles involved in the case after all - and hurtloam wouldn't have helped him even if Foamfollower had still had it left.)
Food for thought: Can Lord Foul be eaten and absorbed? Would this be a good ending? Do you think it will happen?
For sure, it would be an interesting twist on "Thomas Covenant becomes Lord Foul"...