I think that the progenitor of their family is reincarnation of MC. Like their ancestors are trying to “surpass” or get to the “level” of the first Duke. It seems they did everything to make MC stronger as Mephisto is inside of him, there’s a chance Mephisto might be their final boss fight. They need a lot of power to subdue Mephisto, by absorbing a lot of demons. As MC keeps stronger so is Mephisto, so soon he might even come out of MC and you know what will happen. Just a funsies theory! Haha take it as a grain of salt. (My grammar is bad, so my apologies!)
I think that the progenitor of their family is reincarnation of MC. Like their ancestors are trying to “surpass” or get to the “level” of the first Duke. It seems they did everything to make MC stronger as Mephisto is inside of him, there’s a chance Mephisto might be their final boss fight. They need a lot of power to subdue Mephisto, by absorbing a lot of demons. As MC keeps stronger so is Mephisto, so soon he might even come out of MC and you know what will happen. Just a funsies theory! Haha take it as a grain of salt. (My grammar is bad, so my apologies!)