Ryuk had some experience converting people to his way of thinking, and the vampyres looked like the best target around here. He would send small groups of kasantians out as missionaries, bringing new ways of torture and nightmare slave collars.
The goal was to convert the olgogs to the kasantian way. The first step was to get the vampyres torturing others, and in return for the bit of torture, protecting the torturees family.
The next step was to have the olgogs enslave the nightmares that get created with the nightmare collars. This isn’t a far stretch, as it prevents colateral damage and being blamed for harming others. And its good PR to cleanup after yourselves.
The last step in stage one, is to get the olgogs to use the nightmare slave labor. This is going to be done by example, with the missionaries having their own nightmares be seen doing chores and work.
Control is going to be through the rate at which nightmare collars are sent out.