The Battle Hardened comes with three multiplayer maps for Connor to climb, creep, crouch and crawl his way around. These are Charlestown, Saint Pierre and Fort St-Mathieu. There's also three new multiplayer characters available, including a Governor (who Ubisoft say is "influential"), a Highlander (apparently "resilient") and a Coyote Man ("fearsome" and also probably "hirsute").
This DLC will set you back £7.99, unless you're one of those people who has a season pass, in which case you get it 25% cheaper.
If you'd rather have some single-player DLC to enjoy, a three-part campaign called The Tyranny of King Washington is still to come. Here's a lovely video for you: