We're lead to believe that Tasha's master and Ryuwan's master are the same being, but it's most likely both of them are separate beings. And we're not really sure if Tasha's memories are even accurate, since North has a memory manipulator in WH. From what it seems correct so far: Ryuwan's at a young age got send away from his home, since for some reason their family was hunted. He ended up at West's place and was taught how to be a magical gunner. At some point West was forced to sever ties with him and I believe this 'West' is unrelated to WH at this point; I don't think WH even existed at this time.
Around this time North and South proposed a war to exterminate or enslave humans, East wanted to be neutral, while West was opposed to it. North and South plotted against East, so North could steal East's resurrection powers. Soon after this, West and MOST LIKELY Varette were both killed by North. The reason why I mentioned Varette is because of how Ryuwan thought Varette didn't had hostile intent against him and thought that if she's alive, that West would also be alive.
It's possible that North, after obtaining the powers of resurrection, revived both West and Varette. Varette serves North now and according to East, North incarnated West into Tasha.
From this point on, we can't believe anything what's true or not anymore, since we're looking through Tasha's memories. But it's clear that his whole life from his birth, leading up to the disaster of his sister becoming a witch and his sister killing 'WH's strongest, Edea' were all orchestrated by North. Neptis, who's loyal to North, acted as Servant and manipulated Tasha's father. Both Tasha and Aria were adopted and no one knows their true origin, aside from North and possibly East.
Varette, who is probably very close or important to West, is overly protective of Aria. Aria who is led to believe that killing Tasha would unite them somehow. North wants Tasha to be stronger, but would then have to know that Tasha's death curse would have gotten suppressed afterwards. Tasha's memory gets really iffy.