I've been busy editing Blood & Treasure Second Edition for the last couple weeks, a great deal of that time spent on the spells. I'm trying to streamline them wherever possible, and there are plenty of them to edit.
While doing so, it's occurred to me how many of them are really more NPC spells than PC spells. That's not to say they cannot be used by PCs, but they often require more forethought than most players can realistically use (i.e. they are meant for plots, rather than reactions to plots), and they are not as action oriented as most PCs need in dungeon delving.
These NPC spells are useful for GM's to use as adventure hooks or encounters. Here are three such ideas:
Magic jar is a natural, but I don't know that is sees much use as an adventure hook. An evil magic-user hides his essence in some vessel, and uses it as a base of operations for possessing the adventurers or their henchmen, slowly picking them off, one by one, until they are destroyed.
The plan would work as follows: There's a plain vessel in one of the rooms of a dungeon. It's plain so the adventurers leave it alone. An evil magic-user has used it as a magic jar. When adventurers first enter the room, he possesses one of the henchmen, grabbing the vessel before he leaves. He using the form to spy on the party, maybe steal something, maybe put one or more adventurers or henchmen in mortal peril in a way that is not obviously his fault. Since the henchman is carrying the vessel with him, the magic-user can possess others, slowly tearing the party apart from the inside. Perhaps he is ultimately leading them to their doom elsewhere, where he can leap back into his own preserved form.
In this scenario, an adventurer moving on the Ethereal Plane stumbles (literally) across a secret chest that was abandoned there long ago. The chest is hard to open, and probably contains something dangerous that was never meant to be found or released.
The adventurers are hired by a simulacrum (they don't know this) for a mission to rescue the original magic-user. The simulacrum has 50% of the original's experience and knowledge, so she's useful in the party, but not too useful, and there are important gaps in her knowledge that relate to finding the original.
Here's the rub. The original is in mortal peril, and will die before rescued. The original is also evil, and on a very dangerous mission. After the original dies, the simulacrum begins gaining more experience and knowledge, and begins to gain the evil alignment of the original. Now, the simulacrum is leading the adventurers into a trap, where she can sacrifice them to gain the immense power the original was after.