I like the idea, I think there is a distinct lack of items that apply to a specific target.
The upgrade is also pretty cool, and as an active item it could very well introduce some interesting interactions)
( I forgot about the duration aspect so just imagine the item idea is a permanent passive item for everything I've written here below)
With this you're sort of separating yourself and your target more from the rest of the match, kinda reminds me of 1v1 lanes a bit. But as it stands I think the T4 lacks something.
It would be cool if the effect made it so you dealt 25% more damage instead of less to other enemies if the target is dead and vice versa. You could then strategize a bit and think about putting it on the weakest enemy on the team to benefit from the extra damage (As I'd imagine the weakest player is dead more often or you could take matters into your own hands and target them yourself) or put it on the strongest to try and divert attention.
You could also remove the "Take less damage" part for the enemy specifically so it doesn't end up handicapping your teammates by making them tankier. (Would also make sense for you to use this item on a stronger enemy, but put a cap on how less damage they do, maybe 40%? with diminishing returns for each person that gets the item for a specific target).
All in all, my final suggestion for how this item would work would be this
Unnatural Fixation
T3 Vitality Item - 3200 Souls
Imbue on a target enemy hero, for as long as you have this item you take and deal 25% less damage from other enemy heroes.
Compelled Duel
T4 Vitality Item - 6400 Souls
Imbue on a target enemy hero, for as long as you have this item you take and deal 25% less damage from other enemy heroes.
The target enemy hero also deals 25% less damage (Max 40% and diminishing returns for each item that has that enemy as its target).
You deal 25% more damage instead of less while the target enemy hero is dead.
Also hi SkeletonHoney, I like your videos
