Maybe a tumor (as Malfurion sais) or just a bad eye, http://www.wowhead.com/npc=105393/ilgynoth was destroyed and the world tree was saved for now.
(https://www.infinite-guild.eu/upload/smfpics/legion/en/ilgynothmsmall.jpg) (https://www.infinite-guild.eu/upload/smfpics/legion/en/ilgynothmbig.jpg)