Grayseeder

An ex-Illidari scholar captains a modified Legion starship to an apparently barren moon. She tests a brand new type of volatile magic there, but she and her crew hit a snag when a Titan-crafted starship dips into view carrying none other than Illidan Stormrage. According to him, the moon is actually inhabited.

Illidan expects to complete his mysterious Titan-appointed Task and leave as soon as possible. He could not have foreseen the unusual dangers of the hidden alien world, nor could he have expected to confront emotions he has become such an expert at stamping out.

If you want to see theories as to what Illidan is doing post-Legion, this is my headcanon. There are more threats that the Legion poses other than Sargeras, across the Universe, and who better to hunt them down?

There are 22 chapters all posted in one go at 57,000 words and approx 3.7 hours of reading.