A modified LAMENT passive from phantom dancer would actually go really nice on...

Wit's end.

Not exactly as it is on PD currently and I kinda feel like that passive is too strong as is on Phantom dancer in it's current state but this would be a good replacement to the current "UNIQUE: Basic attacks (on-hit) grant +5 magic resistance and reduce the target's magic resistance by 5 for 5 seconds. Stacks up to 5 times for a total of +25 magic resistance and 25 magic resistance reduction."

Say instead you got 3% damage reduction from an enemy champ every time you auto attacked/applied an on hit ability on them them stacking up to 5 times lasting for 5 seconds. Ranged champs only get 2% damage reduction per stack. (only applies to primary target in the case of Hurricane or multiple target attacks.)

What does Wit's end have in common with Phantom dancer? A stand alone Dagger in the build path.

This stand alone dagger could easily be given a build path of Dagger + 300 gold = MOURNING SPITE 12% attack speed Unique passive - LAMENT: auto attacking/on hit abilities apply a stack that will reduce physical and magic damage they do to you stacking additive up to 4 times lasting for 5 seconds.

Phantom dancer changed to 4 stacks of 3% for both ranged and melee lasting for 5 seconds in turn making it have a build up time.

Yes this is a really strong dueling item concept. No I do not think it would break the game given that it only reduces damage from one single target. Yes it would make tanks have to think twice about getting into a prolonged fight in top lane where they are taking a lot of damage.