Okay so first of all I just want to point out that this is the most reaching and most wild theory I've had and it's most likely 100% wrong BUT wouldn't it be cool if:

Athena plants some idea in Lahabrea's (Hephaistos') head. We don't know what this idea is but it's so terrible that he needs to get rid of it and so he decides to completely rip off half of his soul and lock it up forever. This we know from the Abyssos raid, now what if his idea was to create a world in which no one could die, even if they wanted to, they would just be reborn no matter what. I base this on the fact that on Hephaistos' savage raid, he experiments on the players and seems satisfied when he finds a way to make them revive. Hmm, dying and reviving, doesn't that sound very much like he Void? Maybe Hephaistos wanted the whole (still unsundered) world to be like the void shard, but couldn't accomplish his goal. Then, after the world was sundered, Hephaistos found a way to sabotage Emet-Selch, Elidibus and Lahabrea's (his other half's) plan of rejoining the 13th and intentionally caused it to turn into the void. Emet-Selch mentioned that they made a "mistake" out of "ignorance", but perhaps Hephaistos took back control of Lahabrea and knew exactly what he was doing.

As to how Golbez fits into this, I don't know, maybe he's Hephaistos from the future (our present), regretting his decision. Or maybe he could be someone else who became a victim of his decision.
Again, I know this is a wild theory and I'm reaching like crazy here but... What if....