"But there are ways to feed the hunger. Follow me, and I will grant your people more magic than they can imagine." That method ended up being Mana Tap, which Illidan taught to Kael, Kael taught to Rommath, and Rommath brought back to the Blood Elves on Azeroth (attributing the technique to Kael). The TBC cinematic is visually depicting the same meeting Kael had with Illidan in WC3: TFT, as well; as it was telling Kael's backstory. That Illidan has been tapping Fel energies was pointedly shown in Chronicle Vol. 3 (pg. 139-140) as well: "[s]oon, the fel gnawed at his mind, causing him to become more paranoid of his people, believing them to think of him as a failure. Many believed that the fel magic made him unstable."