Trying to think of monsters that treat xp as an attack vector for players. Wights are the traditional monster for this but I was interested in giving players a warning-- like losing small bits of xp over time to freak them out, rather than screwing them over quickly to just make them afraid of wights in future encounters.
Wight Flies - Tiny, moisture seeking-- they suck away memories as they drink from eyes, wounds, or open mouths (1xp per round per fly, DM's prerogative on how many hundreds appear at any location). They leave fragile, ivory-like structures in corpses. If eaten, these will grant the memories of past victims (100 xp per handful).
Grey Mold - Fine, sickly grey, it can grow anywhere-- on men, dragons, liches-- and is difficult to get rid of. Anything in its presence is filled with a hollow sense of despair (lose 10xp per round x your level). If you are hit by something infected by it you will likely be infected too (make a save).
Pot Boy - A thin, sickly looking boy clutching a small crockery. The pot is covered in a filthy bit of rag. If you see it (fail a save) you will no longer gain experience until something is done. Telling someone about it will spread the problem to them (if they make a save they don't see it). Legend tells of whole cities in the dull thrall of a single pot boy-- learning nothing, forgetting, merely existing.
The Gentle Double - A doppelganger that asks permission. If you
say yes and allow it to take your form, you receive a portion of the
memories and experiences of it's current form (500xp x your level). It
then precedes you in cities and rural areas spreading good will and
promises of aid. Also called a Vardøger. Scholars are divided on
whether it is a truly good creature-- pressuring you to do good with its
promises, or a chaotic one-- making promises that it never expects to
The last is not attacking xp, but using it as a reward to give players an interesting choice.