When did Illidan get the title of "Lord"?

[Lucrece]

The game even has the title before his name so it's not like it's an Illidari only thing, so when did this guy began to be addressed this way by people besides the Illidari?

[Fus Ro Yay]

since he became the lord of the illidari i'd guess.

[Nize]

Probably when he declared himself Lord of Outland.

[Hctaz]

He became the Lord of Outlands when he took down the old one: Magtheridon.

[yetgdhfgh]

I guess his followers just called him that and he liked the name so it stuck

[StationaryHawk]

He's been called Lord since before he became demonic at all.

So always I guess? His family is well known, and if we're going by old-school medieval naming conventions, any man of noble birth would be a lord, and any woman, a lady.

Although Illidan was not Highborne, and therefore his family was unlikely to have been nobility. Granted, there's been exceptions, such as Ravencrest.

More likely, it has to do with his occupation of Outland, as others have pointed out.

[Wallmaker Yahzarah]

He became Lord Illidan when he took the Black Temple after taking in control of Blood elves, Broken and Naga with his Demonic forces.

[Soeroah]

Didn't Azshara's forces start calling him that when she brought him into her army?

[Adoxe]

Lord of Outland and Lord of Illidari.

[Rhlor]

Lord of the Illidari

[Thage]

Lord of Outland, Lord of the Illidari. If nothing else, he earned the title by killing the guy who used to hold it and squatting in his digs for a few years.

[Restors]

But Lord of Outland and Lord of Illidari happened way after War of the Ancients. Playing the scenarios as Illidan a mage in Legion, night elves called him Lord aswell... He must have gained Lord title long before.

[Immitis]

rule of cool is the answer you're looking for

[Jawless Jones]

I wouldn't be surprised if Kurtalos Ravencrest gave him the title of "Lord" when he took him on as his personal mage.
And since Ravencrest was a noble himself (though not Highborne), he probably had the authority to do it.

EDIT:

You are hereby marked as one of the Black Rook Hold's own...and my personal sorcerer! As such, you'll be of a rank outside of the Moon Guard, equal to any of their own and unable to be commanded by any of their order!

It wouldn't surprise me if he was given the title "Lord" along with this.

[Sentynel]

When he defeated Magtheridon atop the Black Temple he became the Lord (of the Outlands).

[Aeluron Lightsong]

Lord Illidan knows the way.

[Thage]

But Lord of Outland and Lord of Illidari happened way after War of the Ancients. Playing the scenarios as Illidan a mage in Legion, night elves called him Lord aswell... He must have gained Lord title long before.

It was probably an honorary title granted through his deeds.

[Netherspark]

His followers have been calling him "lord" since long before he went to Outland. It's just another word for master.

[Lumineus]

[Krazzorx]

Because it sounds cool.

When in doubt, rule of cool wins out.