LotR -- Book 1 - Chapter 08 - Fog on the Barrow-Downs

[HerenIstarion]

but not the what or the why

I always had a notion that houseless fëar dwelling in the Barrows where trying to take over and posses the hröar of the hobbits. Whether it was done through jewelry or incantation, seems of less importance to yours truly (though I lean over to incantation), but it answers 'why' question

I certainly remember reading some passage in HoME series which implied such a possibility for 'faded' elven spirits, and thus warned of dangers of communicating with these. I can not provide direct quote right away, but promise to make a search for it as soon as I find the time and reach my books (unless Aiwendil shows up before that and takes me out of my difficulties )