While tracking the master's physical body proved challenging, we were able to locate it in the Nighthold of Suramar. Our intelligence reports that Gul'dan is making final preparations to the Nightwell in order to use its power to usher the soul of Sargeras into Illidan's body.
The only way to stop that from happening is to put Illidan's own immortal soul back.
Take the Soul Prism of the Illidari and venture to Helheim. Defeat Helya and use the prism on Illidan's soul! Return it to Light's Heart.