That story seemed a bit...abrupt. A lot of emphasis was put on the subtle tragedy of the dude forgetting stuff, and then...a monster straight out of Attack on Titan smashed them? Sure, his dementia meant he was too slow to escape, but the Colossal Titan came out of nowhere.
At least the moral of the story at the end fit - when you notice something wrong, it's too late. Seeing signs of dementia means the brain is already fried. They were too late to notice the Colossal Titan. Priestess is too late to notice the curses are seeping in...