Recap: Adventure Time S 2 E 1 It Came From The Nightosphere
Finn is hanging out with Marceline when she asks him to beatbox to her new record, and starts by asking him not to laugh because it's going to be personal. Then she sings about her dad and how he left her. This tears Finn up, and he summons her dad... from Hell. Because it turns out that Marceline's dad is an evil demon who doesn't approve of her way of life and likes sucking out peoples' souls, which is why she sang songs about him instead of inviting him over. He takes Marceline's bass (which is actually their family axe) and walks out when she starts yelling at him.
By the time that a pissed Marceline and Finn have caught up with him, Dad has already sucked up the souls of a lot of denizens of Ooo. And grown really big.
Finn tries to attack him, while he is distracted by Marceline's swearing. Finn discovers that the sucked up souls are literally in him... in his neck... whatever it is
, and tries to free them, but instead drops himself and Marceline's bass to the ground. Marceline is satisfied and turns around to go back home, while the Dad turns back to sucking the souls. Finn, exasperated, turns on the record of Marceline's song he still has on him. It forces Marceline and her Dad to finally talk about it, and Dad starts apologizing... at which point Finn climbs up him and cuts him with his sword, freeing the souls, and immediately sends him back to Hell the same way he summoned him. Marceline is mad at Finn for first embarrassing her and then interrupting her reunion with her dad... but ultimately, not very much, because being near him is exhausting.