Personally, I'm still convinced the DO was looking to know what was happening with Asmodean (they clearly weren't sent by Rahvin, Lanfear seized the opportunity to frighten Rand into joining her side by revealing more than Sammael was after him, so she lied. She had no idea who sent the DO IMO) and extended his power and will in these two darkhounds. I think nothing could kill them because the DO was directly involved (but got twarthed by balefire, because they died before he could do his thing to reform them/keep them alive).

So, we only have Rand's word that these were a "new breed" and at this point of the series it isn't worth much.

It's conspicuously similar to how the Forsaken believed Shaidar Haran a special/new kind of Fade. He's not, he's just invested with some of the DO's will and power.

This doesn't bode well for when Rand breaks the seal. It could mean the Shadowspawn will be far more difficult to kill once the DO isn't as hindered anymore.