Nerak wrote:I'm a bit out of my field here because I'm not much of a
HH fan, but it's been heavily hinted that the
SWs killed off (our "culled") one of the missing legions. I guess it could be a reference to that.
Their other big undertaking it could refer to is of course the burning of Prospero.
Nope. It's neither of those.
The Wolf Cull is in reference to the failed attempt by the Space Wolves to cut Horus off before he could reach Terra, at the battle of Yarant. The aftermath of that battle are in the book, Corax, but there are currently no details about the battle itself.