Read that it explains it, once Curze arrives as usual, it becomes a giggle. Wondering which part of the Heresy to read through now...
I don't think your reading comprehension is very high when you don't understand the difference between power and POWERED.
Abnett's "Legion", because Alpharius is da best of da best amongst all the primarchs, everyone knows it, Horus knew it as well and he was so pissed off by this fact that started all this Heresy stuff. (and Curze allowed M'shen to kill him simply because he realized he could never be as badass as Alpharius)
Until there is at least one Alpha legionnaire left in this galaxy Alpharius will never die, literally. You can kill Ferrus Manus and the Iron Hands Legion is no more, a certain notorious berserker can betray his Legion on Skalatrax and they will never be united again, Curze can be decapitated by the assassin and his sons would never fight again as One, the Phoenician may be corrupted to the marrow and leave his Legion on its own thus destroying the last vestiges of brotherhood amongst these pleasure-seekers. But you can never shatter the Alpha Legion by killing its primarch - because Alpharius is in his every son, they dont experience collapse of military leadership, they cant be beheaded and left disorganised, their morale destroyed, by losing their commanders because ANY of them can assume a command title at any required moment and reshape a particular military division to their specific needs - you know, "cut off one head - two shall take its place" and all this stuff... That's what it's like to be an Alpha legionnaire - they are the pinnacle of badassery, LITERALLY