Blizzard released a video previewing Delves on their official YouTube channel!
Highlights of the video:
There will be 12 Delves scattered across Khaz Algar, exploring various underground themes. In Season 1, a thirteenth Delve will be unlocked, focused on a lieutenant of the Nerubian, Xev'zik. There are Xev'zikk related achievements on the Alpha: Nemesis and Xev'zikk's Spine.
Delves have dynamic objectives that can change daily, with the aim of a Delve always offering you different adventures. You will be able to visit the same Delve multiple times and have different adventures each time.
Chests at the end of a Delve will include things like core progression rewards — upgrade track items, Delve currencies, and a Unique reward: The Delver's Dirigible mount.
Items from Delves will scale based on the difficulty level of the Delve, and will cap out at an equivalent of Heroic Raid or Mythic +5 (in post Dragonflight Season 4 numbers) keys.
The Great Vault will have a new track exclusive to Delves.
Many items like customizable hats and Warbound gear will drop from Delves but, if you're unlucky and don't get them from drops, there is no worry! A new currency called Resonant