The Magic Key is a recurring key in the Dragon Quest series.
Dragon Quest, Magic Keys can be bought at Rimuldar for 53 G. They unlock doors that can't normally be opened but disappear after one use. If you exit the place and come back, the doors will come back, but the treasure will also come back.
Dragon Quest III
The magic key is located on the 3rd basement of the Pyramid. It is the 2nd key acquired in the game. The key is required in order to reach Portoga and progress in the game. In order to get the key, the hero's party must solve a button puzzle which involves pressing 4 buttons in the correct order. The solution is hinted by young boys in the castle of Isis. As of this game, Magic Key's have unlimited use.
It's revealed at the end that the magic keys in the first game were flimsy, and thus broke upon use.
Dragon Quest IV
The magic key is located in a cave west of Aubout du Monde. There is a switch in a treasure chest where the party found Oojam.
Dragon Quest V
In Dragon Quest V you can obtain the magic key in the third act. The key is hidden in the city of Elhaven (Lofty Peak in the DS remake).
Dragon Quest VI
Dragon Quest VII
The magic key is available in Brugeo's Manor after obtaining the Magic Carpet.
Dragon Quest VIII
The magic key is in a chest in Trodain Castle.
Dragon Quest IX
The magic key is found at the Mirage Mahal in Gleeba. It allows access to warded doors. The only doors it cannot open are ones related to the story (i.e. the one in Gittingham Palace leading to the Oubliette)