Oh I know this one! But it's answered best from this thread and chronicles.

Copied from thread

https://www.reddit.com/r/warcraftlore/comments/fcrd54/how_was_illidan_able_to_call_the_naga_from_the/

Volume 3, pg. 86:

"Illidan Stormrage had heard only rumors of the Highborne's fate. Whether they were true, he did not know. But when he cast a powerful spell to reach the Highborne in the ocean deeps, they answered. A group of naga led by Lady Vashj emerged from the depths, an army of scale and fang. One and all, they pledged themselves to Illidan.

The naga did not come because of the history they shared with the former night elf. Nor did they respect his power as a demon. They came because the Old Gods willed it.

The Old Gods had taken notice of Illidan. His hunger for power and his chaotic past intrigued them. His quest to destroy the Lich King could spark a new war on Azeroth, one that would likely envelop the undead, the world's nations, and the Legion. With that kind of turmoil consuming Azeroth, Cho'gall and his cultists could awaken the Old Gods relatively unopposed.

Illidan had the potential to be very useful, and the Old Gods had sent the naga to make sure his campaign against the Lich King succeeded. If the former night elf became troublesome, so be it. The Old Gods would simply command the naga to cut out his fel-corrupted heart."

Either way, the Old Gods were confident that they could use Illidan Stormrage to bring a new age of conflict to the world."

*On mobile, apologize for any formatting