The Grim Adventures of Billy & Mandy

The Loser from the Earth's Core (5x7)

:

When Nergal, the fiend from the center of the Earth, goes to visit Billy's family (his in-laws) for the weekend, he ends up staying with them over a year. When Billy finally asks his uncle to leave, Nergal says he cannot return home because he is a complete failure, having lost at a board game 599 times in a row against his own family, and experiencing constant disappointment elsewhere. Then, Billy and Grim tell him to not worry about being a failure, but to embrace it. In the end, after training with Billy and Grim, Nergal returns home and he finally wins at the board game, but he also makes a crack in the ceiling and his home starts filling with lava.