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So here we have a Shinigami that is or is not possibily the co creator of todays Gotei 13 in terms of the way he has made it so that anyone with enough power can access even greater powers with alot of training due to his application of the Ausachi, these essentially are blank canva's that a Shinigami can imprint their own 'Souls' onto to help further their abilities and understand what their powers are.
So we know 2000years ago Sasakibe achieved his bankai and showed Yama its effectiveness. But what we don't know is when Nimaiya was recruited to become a RG but what we do is know is that after the creation of Ausachi's he was not long after an RG.
Let's look at the fact that he is essentially a blacksmith, so taking that into account we can assume he was a master blacksmith before joining RG, giving that he was probably as old if not 'older' then Yama this gave him enough time to better understand what it meant to be Shinigami and how their powers essentially worked. This could of been the start of him trying to create the Ausachi, he practiced on his own and eventually found of that having the dead souls of deceased hollows/shinigami would help create blank canva's for the imprint of his own powers/soul, but this could of also created a slight imbalance in the way the souls flowed, this then caused the SK to seek out the cause of the problem, of which he found out that it was a new technique that Nimaiya was learning to eventually better improve the defense's of his protectors.
Nimaiya has to be more than just a blacksmith though. The manga hinted at the process of making asauchi earlier. NImaiya said he made asauchi from shinigami souls. Overall being a blacksmith is not enough to do something like that unless blacksmith has a greater scope in SS than it does here. The zampakuto itself probably was not unknown to pre asauchi shinigami. As we have seen the asauchi does not create zampakuto within shinigami, it merely creates a link between shinigami and zampakuto in order to draw the zampakuto out. Just look at ichigo and hitsugaya, both of them had zampakuto and their powers well before they had asauchi. Other shinigami should have gone through that in the past except they they never got an asauchi. Of course within ichigo we did see that there was a point at which he did not have a zampakuto, otherwise it would make no sense for white to become the zampakuto. So perhaps nimaiya was simply looking for a way to help form zampakuto within shinigami and draw their powers to the real world more efficiently and he came up with the zampakuto solution. Well, that seems to be the more obvious answer... If he kinda just accidentally stumbled into it or something of the sort is impossible to guess.....