Disclaimer: I own nothing.
Immortality had its perks. That's what her father had told her, when he'd left her in that abandoned, wrecked city. It would bring you happiness and contentedness, especially once you joined me in the Night-O-Sphere, he had said, before mumbling something about being back later.
He had been wrong.
Marceline knew that now. She had had her suspicions over the years, but now it was confirmed. Nothing was worth it here. She had lost everything she had cared about. Floating over the graveyard, she stopped at each of the graves in turn, each one receiving a tear or two before she moved on to water her own morbid garden.
Jake. Bubblegum. Finn. All dead, lying here in the cold ground, a ground which didn't care for the wondrous people they were in life. Their colours had seeped into the grey soil, as it had taken them, one after the other.
Finn was killed by his wife, Flame Princess, after a long lifetime of burning together. Marceline hadn't attended the wedding, but she and Jake had always come to his grave every night from that day onwards, sometimes talking, but mostly sitting in silence, their minds wandering in the fields of memory, where Finn was still alive and adventuring.
Jake had been the second to go. Especially after the death of his beloved Lady, he slipped into a deep depression that had eventually claimed him. He had loved his pups, but even that love hadn't been enough to save him in the end. Buried here, beside his brother, Marceline knew that the two of them were adventuring onwards into territory she could never follow, on voyages she could never join them on.
Bubblegum. Her world, her true happiness, had been the last to go. Even with her extended life as a Candy Person, she eventually crumbled away and died. She had passed away, nestled next to Marceline, their two heartbeats becoming one in that one terrible moment. How Marceline had shrieked in pain that day. Her howls of sorrow had been a terrible song on the air, which had scared even the aged Ice King in his distant Ice Kingdom; it had caused her father to come to her from the Night-O-Sphere out of pity. The Kingdom had been devastated by the loss of course, and many of the Candy Folk had come to pay respects to their Princess and offered heartfelt sympathies to the broken Marceline. But nothing could compare to, or heal, the hole in the solitary vampire's dead heart.
And now here she was again. Alone, as she always was in the end.
Everybody left in the end. Her heartless father, abandoning her in the terror of a warzone. Kind Simon, who despite their efforts had vanished into the cold, endless labyrinth of his mind. Brave Finn, killed by the woman he loved, and who had loved him. Jake, broken as badly as she was, destroyed by despair. And beautiful Bubblegum, her Bubblegum, finally eaten away by the one thing Marceline couldn't save her from: age. All gone, all beyond her reach.
She sat down in front of the graves. She let the tears flow. There was no-one to watch her. Everyone had gone, and she was alone. So much for immortality.