There are 9 keys in total, which open 9 different chests. There are 4 other special chests which can be opened via other means, but I won't go into those here.
All chests are located on the Dreadnaught and so is the means of obtaining the keys.
The first chest you open (doesn't matter which of the 9) will reward a Calcified Fragment. They also reward Wormspore, Eris Morn reputation and can contain Rare and Legendary Engrams.
Keys can be obtained via the following method:
- First you need a Wormsinger Rune. These can drop from Hive Knights and Wizards on the Dreadnaught (and maybe by other means).
- In the Hull Breach, in one of the caves is a Hive machine which you can insert the Wormsinger Rune. It is in the same room as the Chest of Ur.
- Once the Wormsinger Rune has been inserted, outside the cave, by the opening of the Cabals crashed ship, Hive will start to spawn.
- Killing these will eventually cause a named Ogre to spawn and on death can drop a Wormfeeder Rune.
- It doesn't have to be spawned by you to have a chance at a Wormfeeder Rune, so you can hang around the Hull Breach area hoping that someone else will spawn the ogre.
- The Wormfeeder Rune requires charging and in order to do so, you have to kill 50 enemies on the Dreadnaught without dying.
- Once complete, the rune will transform into one of the 9 keys, which appears to be completely random.
You don't get anything special for opening them all.
There is a very good post on Reddit for these, plus the other 4 chests I mentioned. Can find it here.