Ability to reduce unit sight?

Hi, Is there an ability to reduce unit sight? I really need it... I can do triggers if needed

That is very difficult.
You can give every unit the ultravision abillty and remove it temporaly with triggers. But when a unit attacks the effected unit, it will be reavealed, so you also must edit the gameplay constants.

please elaborate the situation and conditions this spell/ability will have

elaborate? what does that mean? Well im need my hero to reduce another hero's sight, he won't have any shared vision. Condition: Well, no need it should just work when the ability is being cast and end after 20-30 seconds

Hope that what you was asking for... Im not that good at english

Edit:

Thats ok

elaborate = explain in detail
From what i get you want to make a Blind ability like in starcraft but no shared vision. The only ability i know that modifies vision is ultravision and it does notwork for day time viewing and sets night time range to day time range. You mighthave to use JASS.