The Skeleton Key is a key that can unlock any door, realm, case, or prison in the Ghost Zone.
The Skeleton Key is a glowing green key that has has two oddly-shaped teeth and a glowering skull for the head.
In "The Million Dollar Ghost," Vlad Plasmius tried to get his hands on the Skeleton Key when Behemoth appeared. The two battled, ending in Behemoth knocking Vlad out of Vlad's Ghost Portal and destroying his portal. Vlad eventually gained the Skeleton Key, only to be eaten by Behemoth.
In "Reign Storm," Vlad (how he escaped the Behemoth is unknown) uses the Key to unlock the Sarcophagus of Eternal Sleep to obtain the Crown of Fire, which Pariah Dark wore. But Pariah Dark awoke and began to wreck terror on Amity Park. After Danny Phantom defeats Pariah, Vlad uses the Skeleton Key one last time to lock Pariah back in the Sarcophagus.