Slitherine now offers the Golgotha DLC for Warhammer 40,000: Armageddon, an add-on that requires the original game to play. You can find all the details on this page, which is also where you can pick it up. Here's a bit of the description:

Golgotha is an expansion of Warhammer 40,000: Armageddon, set 50 years after the end of the Second War for Armageddon, it is the last great battle of the cycle.

Join the Imperial forces under Yarrick’s command as they secure a footing on the Ork infested world before striking forth at Thraka’s army.

Although Yarrick has brought elements of the Steel Legion in the form of the 117th Armageddon Mechanised Infantry, the player’s core force is focused on the unwavering regiments of the 66th Mordian Infantry.

The Mordian Iron Guard are the bulk of the new units in this expansion, but the player will also eventually gain command of Yarrick himself atop the pulpit of his personal Baneblade tank, the Fortress of Arrogance.