What on Earth happened between XCOM: Enemy Unknown and XCOM 2? The canon fiction says that the player loses the Enemy Unknown campaign and the aliens take control of the planet. But how did XCOM 2's resistance form?

A new Legacy Ops mini-campaign should answer that question, featuring reimagined maps, weapons and armour from Enemy Unknown. The four-mission campaign is the heart of the Tactical Legacy Pack, which releases on October 9. It will be free to all XCOM 2: War of the Chosen players for the following eight weeks. After the free grace period is up, players will be able to buy it for $7.99/£6.49. You need to own XCOM 2 and War of the Chosen to get the pack gratis.

The pack includes an offline challenge mode to let you play through War of the Chosen's daily challenges whenever you like, and a series of escalating missions called Resistance Archives. You also get new soundtrack options that will let you play War of the Chosen with the Enemy Unknown soundtrack or with a brand new soundtrack inspired by the original X-Com: UFO Defense.

"We looked back to Enemy Unknown and thought there was a compelling unexplored gap following the player's 'loss' and before the events of XCOM 2," say Firaxis in a Q&A. "How did Central get back on his feet? How did XCOM even learn of the existence of the Commander? And how damn cool would it be if the original XCOM: Enemy Unknown Gas Station had XCOM 2 gameplay, enemies and updated art?"

The reimagined weapons and armour pieces you unlock in the Legacy Ops campaign will be available in the main War of the Chosen campaign, and assets from the reimagined maps will appear in War of the Chosen's procedurally generated battlefields. Familiar locations like the gas station have been weathered to reflect the aftermath of the Enemy Unknown campaign. The pack also adds new helmets and attitudes that let you further customise a soldier's posture and pre and post-mission walkups.

That's a lot of stuff, so let's break down the features.

Legacy Ops

Four sequential tactical mission covering the period between the end of the Enemy Unknown campaign and the beginning of XCOM: War of the Chosen.

The missions are relayed to you by Central Officer Bradley, recounting the rise of the resistance. The missions are described by Firaxis as follows...

Blast from the Past —"After the fall of XCOM HQ, Central Officer Bradford begins his journey to build the resistance by returning to where it all began."

—"After the fall of XCOM HQ, Central Officer Bradford begins his journey to build the resistance by returning to where it all began." It Came from the Sea —"The broadcast of a fledgling radio DJ is reaching an audience he didn’t intend. It’s up to Central to save the DJ from an endless horde of hungry listeners."

—"The broadcast of a fledgling radio DJ is reaching an audience he didn’t intend. It’s up to Central to save the DJ from an endless horde of hungry listeners." Avenger Assemble —"Join Lily Shen as she works to retrofit the Avenger into a new home for the growing forces of a reinvigorated XCOM.

—"Join Lily Shen as she works to retrofit the Avenger into a new home for the growing forces of a reinvigorated XCOM. The Lazarus Project —"ADVENT is kidnapping key members of the resistance, and it’s up to Central to rescue some of XCOM's most renowned operatives."

—"ADVENT is kidnapping key members of the resistance, and it’s up to Central to rescue some of XCOM's most renowned operatives." Completing these missions unlocks reimagined versions of XCOM: Enemy Unknown armour and weapons in War of the Chosen campaigns.

Resistance Archives

A seven-mission gauntlet made out of procedurally generated maps.

Soldiers gain new abilities and better equipment and enemies grow stronger from mission to mission.

Designed to act like a short XCOM campaign. "By the end you'll be in the highest tier armor and weapons fighting Gatekeepers, Sectopods, and more."

Skirmish mode

Create your own missions by setting squad loadouts, abilities, and enemies.

You can create a save file of the started mission and share with friends.

Weapons, armour, maps

Legacy Ops unlocks Enemy Unknown armour and weapons for use in War of the Chosen campaigns.

The weapons and armour have been remodelled to look more weathered, as though they've seen battle in the Enemy Unknown campaign.

Enemy Unknown map assets have been reimagined. Legacy Ops missions take place on bespoke designed maps built using these assets, but they will then populate procedural missions in the main campaign.

Enemy Unknown gas station confirmed.

Other bits

New helmets and attitudes.

New photobooth backgrounds, colours, character poses.

Two new soundtrack options: Enemy Unknown, or UFO Defense reimagined.

All War of the Chosen challenges available for offline play.

It sounds like a really nice nostalgic free update. I'm particularly excited about the Resistance Archives mode, because it should give us the opportunity to build a squad over a series of missions rather than over the course of a 30-hour campaign. The UFO Defense soundtrack should be neat as well. Keep your eyes on PCGamer.com in the coming week, we may have a few samples for you to listen to.