What was up with Ascian Prime? (general ShB spoilers)

[LineageRazor]

(after all, we've never seen any more Ascians do the same thing since).

Well, that's not entirely true. Emet-Selch seemed to do something similar during his final battle. He calls upon the souls of his fallen brethren and uses them to power up into his final form - and there are SOME similarities between Ascian Prime and Emet's final form (elongated limbs, giant claws). So, it might have been a similar kind of merger, just to a much larger degree since there were a lot more Ascians involved (albiet dead ones rather than live ones).

[Payadopa]

I think I just unlocked that one in Warframe.