Personally, I kind of think Messmer is actually Miquella’s grown up form.

We know there’s a connection between St. Trina and Miquella, and we know St. Trina rules over sleep. Messmer’s name clearly comes from the same root as “mesmerize” or “mesmer” which originally comes from a type of psychotherapy, which is tied to both sleep and compelling memories out of people. Miquella is known for his ability to compel affection, so that tracks to me.

Beyond that, Messmer’s weird gangly limbs look really similar to the arm in the cocoon, so I think it makes more sense that Miquella became Messmer, rather than some kind of switcheroo.

I think Miquella went to the Land of Shadows to secure his godhood and create his new order, just like Marika did, but due to his exposure to Mohg’s blood, he developed into an adult form that’s aligned with flame and blasphemy against the Erdtree, thus the snake. I feel like that tracks with his line referencing his mother and one without light.

The website said that we’re guided to the Land of Shadows by Miquella, but who’s to say it’s not Miquella as Messmer calling us to see if we’re fit to be his new lord?