The Monster Kid and The Demon's Sword (1982)

Kaibutsu-kun's father, the Great Monster King, suddenly complains of pain in his chest and collapses. It turns out that the rival Demon King has placed a curse on him with the "Demon Sword". Kaibutsu-kun and his three companions head into enemy territory, Demon Land. Prince Demokin, the son of the Great Demon King, appears. Out of curiosity, he invites the monsters to the temple where the demon sword is located.