Now, let me just say right away, I have no idea how to NOT take them out so that they open as quest right away, but it is possible, because people ARE very much selling those.
Legendary Crafting writs are your tickets to crown-store house-items, and other rare goodies.
Legendary Crafting writs are obtained by doing regular daily crafting writs: provisioning, alchemy, enchantment, blacksmithing, clothing, woodworking. After completing one of the writs, you will get a container with your reward, among which there might be a Legendary Crafting writ (other things vary from repair kits, material shipments, maps for appropriate materials, etc).
These are tricky and amazing. It’ll ask you to craft a very specific item. You’ll have to make sure it has correct trait, style, color level (from fine to legendary, so keep a reserve of upgrades at hand), etc., and no, it will not adjust to you. Per se I had to buy a motif for 12k, and the quest, obviously, didn’t pay off, but I got my 6 vouchers and I regret nothing.
After crafting the extremely specific item, you take it to the map where your Undaunted quests are, for me that is wayrest, since my starting map was daggerfall.
Sometimes you will have to craft a specific set item, therefor if you haven’t researched things yet – start doing that. And yes, it pays off to do all 6 daily writs now.