NobbelSetting up the Dark Portal meant establishing a connection between Azeroth and Draenor to have the Horde come in and weaken the world for ultimately the Legion to claim. In response to the Horde’s first invasion, the Alliance of Lordaeron rose up. They defeated the Horde, placed those that surrendered within interment camps, and shut down the Dark Portal. Only for it to re-open and the Horde to try again. This time not to conquer Azeroth--their goal was to gather powerful artifacts which would allow them to open up more portals to different worlds. They even gained the assistance of the black dragon Deathwing. He would help the Horde out and in exchange they would hide some of his black dragon eggs on their home world. In time the Alliance of Lordaeron figured out what the Horde was up to and were not about to let them get away with it.
Turalyon, Alleria Windrunner, Khadgar, Kurdran Wildhammer, Danath Trollbane and so many others stepped through the portal to stop the Horde. They were succesful but not before many portals were opened up across Draenor. The already-corrupted world was about to blow so Khadgar had no choice but to shut down the Dark Portal again. They fully expected to die, but did not. Unknown to those left on Azeroth, the sons of Lothar actually survived the transformation of Draenor into Outland. Yet the Dark Portal remained shut and time passed on...
NobbelVashj and Kael’thas set their master free and then it was time to claim Outland as their own by working together with the Broken, lead by Akama. Together they claimed the Black Temple as well as the title of Lord of Outland from Magtheridon. Hiding away from Kil’jaeden wasn’t going to be that easy though. One last chance was given to Illidan to end the Lich King and once again he failed. His allies dragged him back to Outland where he continued working on his real plans, working in secret on his true war against the Burning Legion.
Having the pitlord Magtheridon chained up meant that they could drain it of its blood, transforming orcs into ruthless battlecrazed soldiers ready to fight for Illidan. At the Black Temple, the Demon Hunter recruits started to train more in Illidan's way of fighting, Blood Elves and Night Elves following in his footsteps. However, more troops did mean that they were going to need more supplies. Food wasn’t easy to come by on Outland and water was even more rare. Vashj was sent out to secure recourses within the Zangarmarsh while the Draenei there kept to their main refuge, Telredor, and used its defenses to fend off the naga.
Kil’jaeden eventually realised that Illidan was more then just a nuisance and could pose a real threat to the Legion. While sending out more demons to take him out, plans were made to permanently stop him. The world already saw illidan as the enemy, like the Legion, which was something that Kil’jaeden could use to his advantage. As a bonus, if he could manipulate the Horde and the Alliance into committing their resources on Outland, that would leave Azeroth vulnerable to yet another Legion invasion. They could restore the Sunwell and use its power to create a gateway for Kil’jaeden to come in. All he had to do was manipulate Kael'thas, turn him to their side, which wouldn’t be too much of a problem.
Illidan had taught Kael'thas and his people how to drain energy from artifacts, creatures, and the environment to sate their cravings for magic. Yet these methods only left the elves wanting more. The prince started snorting fel magic and Kil’jaeden whispered to him that Illidan was holding out and that the Legion instead could reveal the true power of fel. In exchange, he simply wanted the prince to abandon Illidan’s side. Over time the trust in Illidan faded away and Kael’thas, unknown to most of his people, decided to ally with Kil’jaeden.