So, there's a spell in the Book of Vile Darkness called grim revenge. It's a 4th-level spell; if the target fails his Fort save, one hand tears away from the arm, doing 6d6 damage, then the hand attacks as a wight until it's defeated. But a successful Fortitude save negates all the effect.
I like this idea, and it seems like the sort of creepy thing the vampires should use--but I don't like how binary it is. If they make their save, they don't feel any fear; if they don't make their save, the consequences are worse than I probably want to inflict upon them. (Particularly since it requires a regeneration spell to restore the missing hand.)
So here's my more progressive take. I welcome your advice. (My differences from the original stat block are italicized.)
Level: Sor/Wiz 4
Components: V, S, Undead
Casting time: 1 action
Range: Medium (100 ft. + 10 ft./level)
Target: One living humanoid
Duration: 1 round/level
Saving Throw: See text
Spell resistance: YES
The caster spits a glowing spider of unholy white light at the target, making a ranged touch attack to hit. If it misses, on subsequent turns it scuttles directly toward the target along invisible strands of force up to 60 feet per round, making another ranged touch attack if the original target is in range.
Once a successful ranged touch attack is made, the spider sinks into the target's hand (roll randomly to determine which hand) and begins a terrible transformation. Immediately, and at the beginning of the target's turns thereafter, the target must make a Will save. On a failed Will save, the target's hand twists and attempts to tear itself free from the arm; the target takes 1d6 damage, drops anything held in that hand, and may not use that hand until the beginning of his next turn. (This damage is considered continuous damage for the purposes of casting spells.)
If the spell does a total of one-quarter the target's normal hit points, the target's hand detaches from the wrist and then, animated and floating in air, begins to attack the subject. [As in the original spell].
Healing magics can offset the damage of the hand ripping free; affecting the hand requires a touch attack against the target (with a +4 to the hand's AC because of its Tiny size).
I like the fact that this version uses all three saves in some way. A high Reflex save will tend to have a high touch AC, which will create a chance of delaying the successful ranged touch attack. (I made it a repeated ranged touch attack, though, because the original spell didn't require an attack roll at all.) The Will save is there directly; the Fortitude comes through because a character with a high Fort save will have lots of hit points, which will allow the character to survive more failed Will saves. (The flip side is that it's not obvious whom Melisande should target with this spell.)
I also like the fact that there are lots of ways to defend against this after it starts. You can try to dump healing into it to keep it from progressing; you can use dispel magic to end the spell; you can pump up the Will save; and so forth.
My main worry is that it may be too complicated to run easily. But that complexity may be worth it to have a really cool spell that strikes terror into the hearts of my players.
I'm also wondering whether 1d6 damage and 1/4 the target's original hit points are the right numbers. My PCs can probably handle a few rounds of 1d6 damage without too much trouble--but on the other hand, if they're busy trying to fight the spell, the vampires can use those moments of distraction to good effect.