Shirogane explains that he's 1,062 years old.
Here's the Wolf Savage Cycle
Here's Toxica with the Mask of Zen-Aku
Basically someone put a curse on someone in order to make them into a mask and where's the Mask came from.
There wasn't any reason given for Zen-Aku's solo enlarging.
Here's the 2 Dukes and Nayzor The explanation for Nayzor's return isn't stated yet but presumably it has to do with the Crown that he lost that turned Toxica into Necronomica.