Destiny: The Taken King has its share of secrets, with the Dreadnaught containing nine hidden chests that require special keys to open. In order to obtain the aforementioned keys, you’ll need to first get a “Wormfeeder Rune.” You’ll want to spawn into the Dreadnaught where you will soon come across an ogre named Blahgrish (which you can summon using a Wormsinger Rune or, more likely, another player will have summoned it as it’s very common right now). Killing it drops the rune you’re searching for.
The rune comes with a challenge for you: get 50 kills on the Dreadnaught. Watch out though, because deaths will reset your progress. Once you complete this challenge, the rune will turn into one of nine possible keys to be used at the appropriate chest.