I just encountered a whisp. I (coincidentally) had my fire spell equipped and cooked the crap out of it, but ran out of magica. I then switched to Chillrend and hacked what was left of it to pieces. On the ground near where it's little summoned whisps or whatever they were had been, I found whisp cores (or perhaps wisp cores). Instead of picking them up though, it said I could hit x to activate them, so I did.

What do these do? I can't find anything about them either on the wiki or on the boards.

They are just reagents, not sure why it says activate, all you are doing is collecting the glow dust from it.

Ah... That explains it. Thank you.