Finn and Jake attempt to save Marceline from the amulet's influence. In a climactic moment, Finn manages to get the amulet onto himself to experience its power before eventually getting it off Marceline.
Watch the climactic moment where Hunson Abadeer explains his motivations for tricking Marceline: [S4E6] Daddy's Little Monster
Hunson reveals he tricked Marceline into wearing the Nightosphere amulet, which fills the wearer with "chaotic evil," because he wants her to take over the family business of ruling the underworld. Finn and Jake attempt to save Marceline from