I feel like caves should be given a fresh role in the game. Once a week, mysterious glowing symbols illuminate the walls of one random cave in the game. The symbols are encrypted in ciphertext, and when deciphered into plain text, lead you to a portal somewhere in the game, transporting you to one of many possible destinations deep in the galaxy. Depending on the destination, it could be an elaborate, crumbling platforming puzzle, or even drop you straight into the cradle of a frenzied horde-mode arena. You won't know until you're on the other side of the portal, and whatever it may be, completing it will give you Nightfall-level rewards. In order to decipher the encrypted text on the cave walls, you must find the key in a chest. It can be any chest (a guaranteed drop like the Exotic bounty), but only on a specific planet for each week, and you won't know without trial and error. Once you find the key, you must then find the cave with the glowing ciphertext. The ciphertext will direct you to a specific section on a planet (like the Buried City in Mars). The portal itself could be at any random spot, and it is up to you to find the portal once knowing its general location. The portal itself is invisible, but when your guardian walks close by, the space around you will start to distort and shake. Once you get very close, the portal will reveal itself through a dark distortion in space, almost like a wormhole. After completing the task and gaining the rewards, you will receive better chances for rare items in chests for the duration of the week, similar to Nightfall's EXP-boost for the duration of the week. If you like my idea, be sure to upvote or comment to increase the chances of Bungie seeing this! I posted this in someone else's thread, but I thought that this idea could use its own topic.