In the first and all subsequent incarnations of Dragon's Lair, the dragon Singe keeps Princess Daphne captive in a magical crystal sphere. It is unbreakable and Singe keeps the key hung from his neck.
In the first issue of the Dragon's Lair comics, it is given the name "Bubble of Helotry". The Wizard apprentice Halvern sees it in the mind of a dead fire drake and then finds it in an old book. It is mentioned to be first used in the ancient Caldera and it had the magical power to dull the mind and enslave those in it "by the dawn of the next full moon". It is how Singe created his harem of princess slaves.
This seems to create an explanation of the differing characters of Daphne (adventurous and feisty in the cartoon series and the comic, and an arihead bimbo in the original game): by the time Dirk the Daring reaches her at the end of the game, it has already affected her mind.
At the end of the comic Dirk destroys the bubble restoring Daphne, and the rest of the enslaved princesses to normal; this means that the deadline to "the next full moon" didn't matter for real.