The Grim Adventures of Billy and Mandy (2001)

S05E07 The Loser from the Earth's Core

During a weekend visit to Billy's family, Nergal unexpectedly finds himself staying for over a year. When Billy finally musters the courage to ask his uncle to leave, Nergal confesses he can't go back because he feels like a total failure, having lost 599 consecutive times in a board game against his family and facing endless setbacks in life. Billy and Grim encourage him to accept his failures rather than be ashamed of them. After some training with them, Nergal heads home, finally wins the game, but inadvertently creates a crack in the ceiling, causing lava to pour into his house.