Inframundo literally translates to "underworld". It's a captivating and central word in the context of Greek mythology, directly referring to Hades' domain.
In the song, it's used when Persephone is tricked into eating pomegranate seeds, sealing her fate to spend time in the "inframundo" each year. This word is not only unique to the song's mythological theme but also immediately evokes the mysterious and powerful realm of Hades, making it very enticing for learners.
This is Destripando la Historia, the Spanish comedy duo who retell myths at full speed, and here they take on Hades. In a couple of minutes you get the entire family disaster: Cronos swallowing his own children, Zeus freeing them, the cyclopes handing out gifts, and Hades receiving a helmet that makes him impossible to touch. The brothers beat their father, split the universe between them, and Hades ends up running the underworld with a black chariot, a monstrous dog, his own river and a ferryman.
Then comes Persephone. Hades grabs his niece, Demeter threatens to stop every plant on Earth from growing, and a handful of pomegranate seeds locks in the six-months-a-year deal the Greeks used to explain the seasons. The jokes keep coming, from Heracles rescuing one intruder and leaving the other stuck, to the deadpan ending that Hades is not really the villain and is very probably your uncle. Because the delivery is quick and casual, it is a fun way to hear relaxed Spanish like se cabrea and mola mucho in context.