I couldn't miss the chance to practice my drawing!
In order to perform this glitch, after Rydia has rejoined as an adult after defeating Golbez, and after talking to King Giott, if Rydia casts the Warp spell, the player will be returned back to the crystal room, from which the player can pick up the Darkness Crystal and leave the room back to the throne room. When the player arrives at the Sealed Cavern, the event will play as though the player had already obtained the Crystal from the dungeon. However, if the player wishes to get any of the treasures inside the dungeon, they will have to do so with one less party member. This glitch has been fixed in the game's latest versions.