The season saw the premiere of the 100th episode of the series, "The Hard Easy" (previously titled "Prince Huge").Adam Muto reported that characters not seen since the earlier seasons would return, with The Earl of Lemongrab, Magic Man, the Lich, Hunson Abadeer and Neptr being confirmed to return.Jake quickly gets bored and wants to leave, but Finn is having the best time and doesn't want to leave... Finn, Jake and Marceline are trapped in an underground cavern with no red in sight.If Marceline doesn't feed, like ASAP, she will turn into a vampiric beast and suck out all of Jake's blood!
But when BMO stays up all night working on the truck, he hears on a spooky radio talk show that Princess Bubblegum is a reptilian replicate.
Following season 5's cliffhanger finale, the reveal of Finn's dad was prominently teased in the promos leading up to the season premiere.
It was also confirmed by the writing staff that a total of eight episodes in season 6 will involve Finn and his dad, featuring some of the show's most cohesive storytelling to date.
Then P-Bubs joins Marceline on a top-secret mission in search of…wait…that’s top secret! Then take a trip with Finn and Jake to the birthplace of BMO. But also hot." Finn and Jake chase the Lich through a dimensional portal where they meet Prismo, who tells them that the Lich wished for the destruction of all life.
This leads Finn to wish that the Lich "never even ever existed." Princess Bubblegum subjects Flame Princess to a series of tests to gauge Flame Princess' fiery tendencies.