My bet is on the Wisps!
afterall they burned down a world tree once before
https://www.youtube.com/watch?v=Ae_gPE2Ox70
and we got a lot of whisps...
btw, Chronicles agree!
Countless incorporeal spirits known as wisps emerged from the forests around Hyjal. They closed in around Archimonde, but they did not attack him. At Malfurion's urging, they instead channeled their energies into the World Tree and ignited the enchantments within.