Well, if you are referring to DotA's Warden Blur then you should do like this:
- Base your ability on DH's evasion (with lower values);
- When the hero learns the ability set its vertex colors like this
JASS:
native SetUnitVertexColor takes unit whichUnit,integer red,integer green,integer blue,integer alpha returns nothing
Where the "alpha" value (which is basically the "visibility"of a unit) should be a value lowered per level (for example 200 if the learned ability is lv. 1, 150 if the ability is lv. 2, 100 for lv. 3 and 50/or 0 for the level 4, so the unit will be almost transparent).
I hope it helps :) Cheers.
