You beat me to the punch. I was thinking of starting a thread that would ask:
If Harry became an immortal, who or what would he be?
My first thought is that Harry would take some immortal's mantel on Halloween, probably by killing them. That would be easier said than done, seeing as Mab, Odin and the Erlking gave Harry the opportunity to lead the Wild Hunt. He had to earn it, but it wasn't like Harry planned it that way from the start. It didn't occur to me that Harry might be able to create a new mantel of his own.
From Eb's perspective, this is what he was warning Harry about, something that looked too good to be true.