One of your first trips into Torghast will give you a key to show to Ve’nari. She sends you back into Torghast. But, it’s not to the wings with all the mobs: when you first portal into Torghast, turn right and go to that door. It leads to the Runecarver. He’ll give you some quests (that yes, send you back into Torghast) and leads to a quest for him to collect a bunch of soul ash. He is the one that will build you a legendary item, but his quest description isn’t entirely clear as to how to go about it.
Once you have done his first quests, he’ll send you out to recover a memory. These memories are the patterns for the legendary effect. You can see a list of them if you click on your Adventure Guide button and then the Powers tab. Hover the mouse over each of the powers, it will tell you what the power does and where it drops. Some drop in dungeons, some in Torghast, some in the raid. If its from a raid, it doesn’t matter what difficulty of raid it is, the memory will still be the same.
The tooltip will also tell you what items that legendary power can be applied to (e.g., “shoulder, finger” means that you can have a legendary item crafter for either your shoulders or as a ring). This legendary is its own thing; it’s not an enchantment you add to some other piece of gear, but it will be a Ring of Celerity or Shoulders of Dusk and Dawn.
You need 4 things in order to build a legendary:
You take all these items to the Runecarver. It will open an interface, put the base item in the center (so he knows what you want it crafted on), then select the memory you want in the box to the left, and the two missives in the other two boxes. Press create and watch the cut scene.
Side note for crafters: once you unlock the Runecarver, head back to your profession trainer. They will now be able to train you to make the special base pieces at Rank 1. Each time you build a rank 1 item, you will get experience toward rank 2. You have to build 15 rank 1 items to unlock rank 2, then 15 rank 2’s to be able to do rank 3, etc. That’s why the base items are so expensive on the AH. I think there are guildies who can build all the different rank 1 pieces, but only a few rank 2’s. And several people who can make missives. Ask in guild if you’re interested, you should expect to provide the raw materials but they may be able to help out.
You can build legendary items at 4 different item levels, from ilvl 190 at rank 1 to ilvl 235 at rank 4. You need a base item of the right rank and the appropriate amount of soul ash (1250 to build a rank 1 and 5150 to build a rank 4). However, the legendary effect doesn’t change, only the amount of secondary stats you get. Rank 1 legendaries are fine for most purposes.
If you build a rank 1 legendary, you can upgrade it later. You’ll need the higher level base item and the difference in soul ash. So, for example, rank 2 requires 2000 soul ash, but if you are upgrading you get credit for the 1250 it took to build the rank 1. So, you just need the Rank 1 legendary, the rank 2 base item, and 750 additional soul ash. You cannot change memories or missives when upgrading.
What legendary should you build? It should be one that fits your play style and what you’re trying to do. Number crunchers have determined the ‘best’ for certain circumstances, and that’s always a good place to start looking, but make sure its something you can make use of.