https://www.gamespot.com/articles/destiny-2s-warmind-expansion-brings-in-a-new-horde/1100-6458467/

During Bungie's reveal livestream for the Warmind expansion for Destiny 2, the developers showed what players can expect for the Guardians' return trip to the planet Mars. With a long-hidden threat recently unearthed under Mars' ice caps, the Guardians will team up with Ana Bray and the long-lost AI Rasputin to prevent the spread of the lost faction of Hive. Featuring a new story and area to explore in the Hellas Basin, players will gather new weapons, Exotics, and take on new bosses in the Warmind expansion--which launches during Destiny 2's Season 3 event.

In addition to new activities and locales to explore, the new expansion also comes with a new community event that will unlock after finishing the main story. Called Escalation Protocols, these unique world events will trigger a new horde mode-style mission that players can take part in solo or in a group. Unlike other world events and missions on other planets, these new Mars set events aren't on world-timers and can be activated at anytime. Completing these events will garner new items and gear specific to the Escalation Protocols. While you have to finish the main story on your own to trigger them, you can still join in on other players fighting through their own event.