After the defeat of Archimonde at Hellfire Citadel on the alternate timeline world of Draenor, Gul'dan is pushed through a portal to Azeroth of the main timeline in order to open a way for the Burning Legion to invade in far greater numbers than in the War of the Ancients thousands of years earlier. While exploring the hidden vaults of the Broken Isles, Gul'dan finds a dark crystalline prison containing the corpse of Illidan Stormrage, the former Lord of Outland, who was slain during the events of The Burning Crusade.
Source: Wikipedia, "[en.wikipedia.org/wiki/World_of_Warcraft%3A_Legion World of Warcraft: Legion]", available under the CC-BY-SA License.