Why do you think Mortuga was opening the cauldron?
I’ve only played through the game once, so I apologize if they did give a specific answer, but from what I can recall, they didn’t. And I prefer it that way.
The game kind of compares mortuga to the killer, >!Gripp.!< It’s mainly the “lying” aspect that they seem so frustrated by at the end of the game. They, themselves, are a liar in similar ways to get a specific goal. The obsessiveness they both have with different versions of “her.” There are comparisons to be made, at least to an extent.
I like the insinuation that Mortuga was maybe going to use the mask himself, even if we’ll never truly know. Speaking to the mermaid is treated as a force we really shouldn’t be messing with, and our lack of knowledge can be okay. The idea that, in the end, even the victim himself decided to mess with powers outside our understanding, especially for possibly ‘selfish’ reasons related to Felicia, is very interesting to me!! Either that or he just wanted to hide it lol