In World of Warcraft, for some wow gears, if the epic means mythic, then the legendary one is a symbol of the best. Only the best weapons or gears have this color. For a long time, all of the Azeroth players are 
trying to search for the suitable legendary gears, but only the best and luckiest players can get it.
In the'60s, the legendary weapon: Thunderfury, Blessed Blade of the Windseeker, and Sulfuras, Hand of Ragnaros, not only needs a strong team but also needs a large number of wow gold.
Warglaive of Azzinoth(main&off hand) is regard as the best weapon for Rogue and Warrior, which dropped from Illidan in the'70s, and it has been the best witness at that time. However, because of the low drop chance 
and the limit of main and off hand, what's a big pity that many players only get one. 
As a legendary weapon, it represents a top team's hard work, whatever this drops from boss directly or crafts out, but the best memory will remain with these weapons. 
Maybe Blizzard wanted to let more players enjoy the charm of the legendary items. They added the legendary cloak in MOP and legendary ring in WOD patch.
However, the legendary cloak has been the masterpiece with the patch of WOD, and the new legendary ring has appeared through completing a series of quests. After more than two months, most of the players has completed all quests and get the legendary ring. Upgraded to ilvl795 by crystallized fel(purchasable by valor points or dropped by Archimonde ). You really cannot imagine what it will be a strong team when all of them own ilvl795 legendary ring.
The legendary ring questline cannot complete in Legion. If you have started the questline before the pre-patch, you can finish it before Legion hits. However, you cannot pick up the questline during the pre-patch and start it.
It's very lucky to own these disappearing gears, if not. You will regret that.