His aim wasn't to rule, only to help mankind to reach the next evolutionary step by granting upon a selected elite the gift of immortality and matchless power.
Fixing a centuries old system, however, was much harder than just overthrowing a ruler. The Master needed not only the resources necessary to turn regular humans into Eldritchs, but he also needed to find a way to make the transformation safe and permanent.
So far, all he had achieved were hybrids and no matter how powerful they were, until their condition stabilized, they would be a treat for themselves and for others. Vastor himself needed the Yggdrasill staff to contain his Abomination half.
He couldn't move his operations to the next step until he was sure that the gift he wanted to share with humanity wasn't actually a curse that would doom them as it had already happened to the fallen races.
