I presume that is literally Illidans Inner Demon. If you've read the Illidan book or paid extra attention in the DH starting area you find out all Demon Hunters have an Inner Demon they must control and contest with. Usually DH simply die or ...explode like what happens to that one DH in the DH starter zone when you give him the Tome of Secrets but I imagine Illidan no longer had the strength to contain his Inner Demon after being drained in Maeivs Crystal for an extremely long time so it was powerful enough to leave him and we have to kill it.
Or I could be totally wrong and it's some sort of Shade of Illidan controlled by Sargeras.