Games Workshop is wrapping up their Exemplary Battles series with a new scenario for you to download. It's the Iron Hands vs. the Blood Angles. Who will win out? That's for you to decide in this The Horus Heresy scenario.
From the post:
The period immediately after Horus rebelled was a maelstrom of confusion, betrayal, and tragic misunderstanding. Nowhere was this apparent than on the isolated world of Canopus.
In this month’s free Exemplary Battles download,* a battered group of Iron Hands survivors from Isstvan V encounter the duty-bound Blood Angels defending the Canopus refineries, with neither willing to budge.