This quest is not available in game.
Archmage Angela Dosantos at Light's Hope Chapel in the Eastern Plaguelands will make Frostfire Leggings if you bring her the following: 1 Desecrated Leggings, 20 Wartorn Cloth Scraps, 4 Mooncloth and 2 Nexus Crystals.
As a Watcher, I was allowed access to many of Medivh's personal effects. Among those items, I found the tomes most enlightening. While the majority of the books were filled with - to put it bluntly - tripe, there were some gems. One of those tomes documented the creation of a set of armor fit for an archmage: Frostfire.
<Angela points to her head.>
It's all in here and I am willing to share the fruits of that knowledge with you as long as you are able to provide me with the necessary materials.