As far as I was aware, Eidolon was a significant part of the current Emperor's Children command structure, and had taken part in several post-heresy events, including the 1st War for Armageddon. As far as I know, this is included in previous published GW fluff, but I'm not sure where.
However, barely after the beginning of the novella, he's beheaded by a wildly emotional Fulgrim.
This appears to be a fairly significant ret-con.
So, is this some purposeful "So... if Eidolon is dead... WHO's been pretending to be him all these years??", or a Lucius-like resurection, or some other 'Chaos Did It'? Or is the whole novella a 'It was all a dream...'?