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Deck of Many Stings

When a card is drawn from this deck, a portal momentarily opens nearby, just long enough for an attack to fly through that targets whoever removed the card from the deck. This attack is performed with a random weapon type, and the bonus for this attack roll is +20. Critical hits and damage rolls occur as appropriate for the randomly selected weapon.

Piercing Spell from Ultimate Magic already achieves something like this, it treats spell resistance as 5 points lower as a +1 level increase.

Unless you're adamant about letting your players cast Phantasmal Killer with an effective +15 against Spell Resistance (not a terribly effective use of a 9th-level slot, though), you might just want to go with what's already written. If you're still looking to keep it, you could at least reduce it to, say, a +3 to level.

The earlier posts about amnesia through Bestow Curse got me thinking, so here's a little something that doesn't just strip you of all your class features forever. This one'll be infinitely more tricky in a real-life setting, however.

Curse of Imperfect Recollection
The victim of this curse believes they are actually a class with similar class features (if they have an archetype that lifts features from another class, such as Iroran Paladin, this class is preferred), or a class they have retraining synergy with if no suitable choices exist. In the event that a character has levels in multiple classes, the GM picks only one class to be "replaced" by this curse.
Their character sheet is adjusted as if they had retrained into this class. The GM keeps a copy of the character's previous sheet, with their true class represented as well as hit points and any other properties that change with their class.
Successes and failures, either made by or against this character, are reported to the victim as if they were their new class, but effects are actually resolved using their true class.
In the event that the character tries to benefit from a class feature that they don't actually have, treat it as a failure unless doing so would produce different effects than not having the feature at all.

Here are some others.

• Affected character (somebody with an animal companion or familiar of their own) perceives all house-pets, animal companions, mounts, and familiars as hostile.
  
• Affected character (somebody with a focus on social skills) refuses to speak to others without first assuming a disguise that would (hopefully) obfuscate their race.
  
• Affected character (Antipaladin or anybody else with a code of conduct) always acts to prevent harm to sapient life (or, if not possible, lessen it), even if doing so would run counter to their alignment or their deity's tenets.
  
• Affected character (Casters, or possibly somebody who could steal from their allies) compulsively eats from any spell component pouches they have access to. The character takes two minutes per spell level to clear out a pouch. This time does not have to be consecutive. For every two minutes, the pouch fails to provide material components for one spell level the owner is capable of casting. The character is affected as if they had eaten all the material components for spells of affected levels prepared by the owner of the spell component pouch, including suffering from poisons or anything that is otherwise not suitable for consumption. If the owner of this spell component pouch is not capable of casting spells, assume it contains components for a random selection of 0th-level and 1st-level spells.