I was skimming through Fantastic Beasts and Where to Find Them the other day for a fic I'm writing and came across this:
There's been speculation that there's a connection beween Dementors and Lethifolds, but now I'm wondering if what we know as the veil was actually a Lethifold being studied at the Dept of Mysteries, and that Sirius didn't fall *through* it so much as... yeah.
This would also explain why Remus, above all others, was the one to firmly tell Harry that Sirius wouldn't be coming back... as a DADA professor and something of an expert on magical beasts, he would be one of the first to recognize a Lethifold, no?