WTF is The Mountain?

Adventure Time is an unabashedly weird show and I love the hell out of it for it. It has so much lore and so many characters, yet the writing is so good that, even with all the wild stuff happening, I can understand what is going on for the most part. Until now.

What the hell is going on with The Mountain? From the moment Finn and Lemongrab stepped into Mt. Matthew I felt like I was watching something out of Twin Peaks. I am so confused. Why does Lemongrab want to be absorbed into Matthew? What was with all the trippy imagery? Why was Finn the only one who could follow him?

This is the first time I have been left confused. Yeah, I’ve watched trippy episodes (mostly courtesy of the Magic Man or ones like the comet one where Finn was astral projecting) but I always understood more or less what was happening. This is the first time I have no clue what was going on.