@Dushadushadusha & @Witness,

Zehir specialization - Master of elements give 2 bonuses.

1. For any killed stack of his own he gets a group of elementals following this rule:

1
40
1

This means he will gain 2 elementals on levels 1-39 and 3 at levels 40-79

2. For every level gained he gains bonus spellpower towards "Summon Elementals" spell and "Conjure Phoenix" as follows:

1
3
1

This means 1 spellpower per 3 levels gained.

Zehir is the most powerful Summoner in the game and as such he has his own niche. Due to his specialization he is very predictable in PvP but is a good counter against heroes that lack abilities to cope with Summoned units. His disadvantage in PvP is compensated by having very easy creeping and adventure map progression through the whole game.