Secondary skills in Heroes 3.

to gamer-kirill
There are NO heroes with a native spell called "lightning."

Certainly, they meant Ciel with the arrow, Solmyr with the chain lightning, or Alagar - with the ice lightning...

Added 2 minutes later

and if you calculate it, what percentage will this give up to level 20 with an expert?

Expert gives +15% to the experience gained. The level is irrelevant here, as there are no specialists in this skill.