Ok, the way to summons these guys is :

1. Collect twilight armor pieces. You need all 3. (Cowl, Mantle, Robe)

2. Collect at least 10 Encrypted Twilight Text.

3. Turn the texts into Hermit Ortell located at 67.1, 69.7, in the cave in Silithus. Need at least 10.

4. Within 24 hours of turning in the texts, you'll get a letter from Ortell in the mail. Along with a letter, you'll get a package of documents that were supposedly translated from those texts.

These will vary from decrypted texts that contain news about the Twilight Cultists, buff scrolls, and sometimes you will get a scoll on how to create a crest of beckoning of each of the four elements.

5. Once you get the crest of beckoning for thunder. Create it and head to a lesser wind stone.

6. Put on all 3 pieces of twight armor and activate the lesser windstone by clicking on it.

7. The Hoary Templar should spawn.

If you dont have the crest in your inventory or enough Texts to turn in then you may still be able to summons the Hoary Templar by clicking on the lesser windstone after putting on the armor, its just a 1 in four chance you will get him instead of the other 3 elementals. The crest helps to insure which elemental you spawn.

Hoary Templar = Air Elemental templar

how do you summon him for this quest?

The repeatable Field Duty quest can randomly lead to this quest.
Field Duty quest for Alliance
Field Duty quest for Horde

Note: You'll have to do a quest chain starting with http://classicdb.ch/?quest=8284  before you can turn in 10 Encrypted Twilight Text to Ortell.

Hermit Ortell says: Leave at once, stranger! I have no business with you. =/