Grukar is an orc who found Thrall, having entered a portal in Caverns of Time, who introduced himself as Thra'kash a warlock who was sent to assist him with a secret mission. Grukar took him to a camp where Orgrim met with Durotan and Draka who told him about Gul'dan's betrayal. Orgrim sent them to a secret location and Grukar with Thra'kash were to accompany them. In fact, Grukar was the one who summoned three assassins that killed Durotan, Draka, their wolf companion, Sharptooth and in turn Grukar.
In the main timeline, an unnamed guard was described to Thrall by Orgrim to have indeed summoned the assassins, thus the event occurred the same except of the presence of Thrall.