Lost Brothers

The Lost Brothers is one of the earliest recorded Imperial xenocidal purgation campaigns undertaken by the Obsidian Blades Space Marine Chapter. It is notable for being one of the Chapter's shortest conflicts as well as having the details of this campaign purposely obfuscated for unknown reasons.

History
238.M38) === During one of the Chapter's shortest conflicts, they were tasked with the complete extermination of a very Warp-sensitive xenos breed about which exists effectively no record. During the earliest days of the conflict, a Warp Rift opened that split the planet in two.

Nothing is known about exactly what happened next since even the Chapter's own records are almost completely non-existent, but what is known is that the Warp Rift closed shortly after it opened. Next to nothing remained, with only a small debris field of metal and rock teetering on the fringes of the warp rift's reach.

As many as four hundred Astartes - the true number being lost to time - were on the planet when the Warp Rift opened and none made it off the surface once it did.