Lightning is an electrical discharge caused by the difference in potential (voltage) exceeding the break down voltage of the atmosphere that separates to two points.
The difference in potential is caused by the accumulation of positive or negative charges on either or both points — one being positive and the other negative.
So, literally the opposite of lightning is no the condition of there being no difference in potential between two points. And, that pretty much looks like any day without lightning.
But, I suspect that is an unfulfilling answer to you. So, getting less literal. The opposite of lightning might be the accumulation of positrons — which are anti-matter equivalents of the electron. They carry a positive charge, instead of a negative charge. Since they are charge carriers, a difference of potential is established that can result in the atmosphere breaking down — just like lightning.
Except, the positrons colliding with matter annihilate an equal mass of matter. So positron electron collisions result in zero mass and some radiation. In generally accepted theory, positron atom collisions result in nothing except a photon emission. But, if you accept that idea that that idea is always right, then you could speculate that the atom became .... unstable matter as a tiny portion of an atoms sub atomic composition is annihilated. Assuming you are looking for something interesting for your story, pretend that gases caught in the discharge start decaying. Trees and goats struck by the opposite of lightning blow apart and their atoms start to decay atomically, becoming radioactive while they breakdown into subatomic junk