I believe the appropriate reaction is "ohgodwhy" among various profanities.
Uploaded onto Black Library's blog, the publisher displayed a number of artworks to upcoming novels and promote their products. These consisted of an image of Archaon the Everchosen, the cover of the upcoming Horus Heresy: Vengeful Spirit tale and an image of a Grey Knight fighting against hordes of daemons single handedly. Displayed above, the image bore the name "MORTARIONS_HEART.JPG" with a reaper figure with wings lurking to the right and the Grey Knight in question being
This was made known to people some time ago that Draigo would be given his novel, worse still it would be based upon an incident where he casually punched out a primarch and performed open heart graffiti on him, but some had thought it had been cancelled. Originally scheduled for August 2013, it seems that the original date was only off by a few months.
Along with it being based upon a heavily criticised and mocked incident with a character widely regarded as being poorly written, the cover itself does not inspire confidence in the creators. Mortarion, now a daemon prince of of Nurgle, is depicted as barely having changed from his mortal form. Whereas previous daemon primarchs were shown to be twisted, corrupt titans barely resembling their mortal bodies, Mortarion has barely gained little more than a few basic cosmetics. Not a good start.
The actual release date is yet unknown, but it's an unfortunate reminder that Games Workshop hates its audience and is utterly deaf to criticism.