Every so often studios will put time into crafting alternate reality games. Some of them don't go so well, like the time Capcom scattered fake body parts throughout a major city, but some are pretty awesome. I think this recent Destiny 2 stunt fits into the latter category.
For a few days since the launch of the recent Warmind DLC, fans have been cracking away at a puzzle that eventually led to a set of coordinates. This was a full-on investigation, with a lot of the credit going to the subreddit Raid Secrets, and the redditor Randomiser who solved the cipher. When an intrepid group went to the location they found coins, a letter, and a full-on Valkyrie spear -- the weapon type featured in the Warmind DLC.
It looks like the activity was the brainchild of Rob Gallerani, design lead at Vicarious Visions, who collaborated with Bungie on this particular add-on -- the two-decade old studio has worked on tons of properties and most recently handled the Skylanders series alongside of Toys for Bob. Nice one Rob!