Yup. I found it. Me and Aqua ( i shall not use personal names ) were at the bookstore and found somthing. AT comic issue #1.

On pg.66, PB&Finn talk about how they killed the Lich, this is what they said: PB: Did you get him?( talks about The Lich ) Finn: Well, I guess you could say that The Lich should've worn a wide-brimmed hat today, because he's spending an awful lot of time.... (next pannel) IN THE SUN.(end of charachter's talking) finn says "in the sun" in big red uppercase  letters, that are in the same font as the AT logo with the sword through it. Finn says it with a big open mouth and huge eyes

.Here is why i know it's vital. Today, in school we learned about how the author sorta "whispers" to the reader who is reading his/her book. that's what the author did with this book. he whispers to us. The reason i know that this is vital, because at the bottom of the page, in very light blue letters, it reads:BEHIND THE SCENES FACT: All of Adventure Time, the show AND the comic, was leading up to Finn's part in pannel three. ALL OF IT.

This may seem cheesey, like, i know what you're thinking. because of my Simon Petrikov theory in a recent blog post, you don't belive me. But i've never been more serious about anything in my life. Well, basiclly AT is my life, and i'd never lie about it. There it is. The Warden ( nickname i gave to Pen Ward )'s secret revealed tell me if i'm wrong.