The patch has only been on the test realms for a few days but we can already find a couple of information on the questline to get the legendary daggers! The quests below are the 4 first quests found in the game files, they will most likely be followed by more of them.
Proving Your Worth
Objective - Pickpocket a cryptomancer's decoder ring from a Twilight Sorceror at the Azure Dragonshrine within the Dragon's Soul raid encounter.
Description - Hello, [name]! You've earned a reputation among the flight as having - shall we say - 'nimble fingers.' We need someone of your skills for a matter of utmost delicacy. Before I reveal more, we need to know you're the right [man/woman] for the job.
When the course of battle takes you to the Azure Dragonshrine, I need you to pickpocket the Twilight Sorcerors until you discover a cryptomancer's decoder ring. Are you up to the task?
A Hidden Message
Objective - Charge your Cryptomancer's Decoder Ring and use it to decode the Corastrasza's Singed Cipher.
Description - Corastrasza, currently stationed at the Vermillion Redoubt in the Twilight Highlands, has intercepted a coded message of grave importance. I'll let her explain the full situation, but we need your expertise to unravel the cipher!
Take your decoder ring to an ethereal thurmaturge in your capital city - he should know how to charge it. Acquire the encoded cipher from Corastrasza and use the ring to decode it for her. Hurry! Time is ticking...
Objective - Speak to Mostrasz at the Vermillion Redoubt and fly to Ravenholdt Manor together.
Description - If the egg is in Ravenholdt Manor, we can't risk torching it in an all-out assault. I'll need someone slippery to get in there and find out for sure. That's you, [name].
Mostrasz will help you infiltrate the compound. Speak to him just outside and fly to Ravenholdt together. Good luck!
To Catch a Thief
Objective - Infiltrate Ravenholdt Manor and locate the missing black dragon egg.
Description - Just beyond this cave lies Ravenholdt Manor. You need to get inside without being seen!
*Mostrasz places an enchantment on you.*
There - now I'll be able to keep an eye on you and communicate as you venture into the compound. Now sneak into that manor, locate the egg, and don't get caught...