not completely done.
This is the idea:
During the course of the Beast Yeast arc, Shadow Milk revealed that he was holding Pure Vanillas daughter (Moonflower Cookie) hostage, the two settle on playing rounds of chess to decide on deals and how to settle things between each other.
As of currently, Shadow Milk has the right to possess Pure Vanilla during certain parts of the day (mainly dinner and night time), while Pure Vanilla has the right to resist these attempts.
Pure Vanilla got his daughter back (partially), she's pretty much been placed into an eternal sleep like state, something that PV is trying to reverse.
Another odd thing that Shadow Milk managed to win in these chess matches is the right to invite anything inside the castle, this is something that PV didn't think anything of until he got home...And when the corrupted ghosts and spirits began showing up.
(Wanted to clarify that this deal included that PV or Moonflower CANNOT move out of the castle)
Not only does he haft to contend with Shadow Milk Cookie, but also increasingly dangerous ghosts and spirits settling into the castle (needing to protect himself and Moonflower from these things).
Now, he must pick up a lost art within healing to deal with these spirits. He only knows basic charms and temporary spells to expel or ward off the ghosts and spirits as all of the knowledge was passed down orally (word of mouth) and it seems like anyone within this field are long dead due to the dark flour war.