Surprise! Lightfall just added a secret Exotic mission to Destiny 2's oldest area

gamesradar.com:

The first weekly reset of Destiny 2 Lightfall just added a secret Exotic mission which unlocks the new Vexcalibur glaive, and you're going to want to see it for yourself.

Of all places, the new Exotic mission starts in the EDZ, one of the few remaining destinations from the launch version of Destiny 2. Head over to the EDZ's Gulch and you'll find the area dotted with six Vex data clusters. Collecting one will refresh a 30-second timer and reveal another number on a keycode.  Here's a short, spoiler-free video from Ehroar (opens in new tab) showing where to find them all. 

Once you collect all six, take your completed keycode and head to the southwest corner of the bend in the Gulch, just behind a rock near one of the data clusters. You'll find a short and unassuming cave with a Vex Harpy resting in the back. Interact with the Harpy to start a new mission which rewards you with Vexcalibur. 

I won't spoil the mission itself, but I will say that it looks like we're going to run this thing several times to unlock Vexcalibur's catalysts, much like we did for Revision Zero in the Season of the Seraph. The new glaive can be crafted just like Revision, so you'll want to break down the first copy you get and then use its pattern to craft a fresh one at the enclave from The Witch Queen, so you can continue leveling it up. 

At first blush, Vexcalibur seems much stronger than the original Exotic glaives. It's a Void weapon with a special, low-duration shield that grants Void overshields to you and nearby allies whenever you block any incoming damage. While you're protected by this overshield, you'll deal increased melee damage and totally refill your shield (with a 10-second timer) on every kill. It's sort of like Void's Devour