Grey Knights / Craftworld Iyanden vs Dark Angels.
Two seperate strike teams descended down to the forge world of Minoras IV. One company of Dark Angels sought the source of taint on this world and a hope that it would lead them to one of the Fallen. One company of Grey Knights came to investigate the strange readings that had been recieved from this world. What both found was a huge forge operated by the Dark Mechanicus to manufacture daemon engines and weapons of mass destruction.
Both sides deemed to best to report back to their commanders what had been discovered so that a larger force could be activated to bring the forge works down. On their route back to the drop sites, the two forces detected one another. Wary after what they had seen on this world, neither were prepared to take the risk and opened fire. The Dark Angels had a better position and held their ground in the ruins of a derelict factorum, forcing the Grey Knights to advance across the open ground.
From nowhere the Grey Knights found unlikely assistance. Eldar troops from the Iyanden Craftworld emerged from the webway and attacked the Dark Angels, much like they had on Fangol only days earlier. But even their aid was not enough. The Dark Angels outnumbered their foe and had better equipment, and a more defensible position.
As the last of the Grey Knights fell to the ground dead, the Eldar mysteriously vanished like smoke on the wind. The Dark Angels left them where they fell. Should anyone investigate they would assume it to be the work of Chaos.